Choosing a good file name is really important. In best case, a file name will describe your whole file in two/three words.
If you're uploading a skin, it's good to write down what weapon skin you've created. If it's original skin without using references to any weapons from other games or real life weapons then naming is not so important.
Just make sure that you do not write what you're replacing.
Examples:
"Medieval sword for machete" is okay,
"New machete skin" is not okay.
The first case helps people understand that you've made a skin that looks like a medieval sword, whereas latter case just says that it's a new machete skin, which will lead to people thinking that you made a skin that also looks like a machete (even if you made a sword)

If you're uploading a map, be sure to write the map type (i.e. de_,ctf_,as_,dom_ and so on) as well as describe your map in short. Writing your initials is alright but there's no need to write your username. People can see who uploaded the file when they click on it. Try not to include special symbols as they are unnecessary and annoying.
Examples:
"[RM] ctf_beachforts" is alright,
">>>Raining Mammoths<<< new epic map ~~~" is not.
In first case the tag provides information on who made it, without taking all of your attention. Most people don't think too much about tags as they just click on the file to see it. In the first instance there's also map type, so user can know what kind of map it is and gives a little information about the map (tells that it's a map with a beach and multiple forts)

If you're uploading a lua script, be sure to include only the name of your script and if it does only a single thing (or a single type of thing) it's best to write exactly that.
Examples:
"New radio system" is ok,
"Raining Mammoths' best lua!!!" is not.
The same thing applies to most of the names of uploaded files.

File name has to say what you created in a short, understandable way.

If you're uploading a spray or misc file, make sure to write down exactly what is it that you're uploading. As far as I've seen, most names in these sections are okay.

Writing a good description is not an easy task. Nobody likes tons of text (oh the irony) so try to stick to the point. Unfortunately, some files require big amounts of text so make sure to use more tags

Using text formatting styles such as bold, italic and underlined is not needed. They are meant purely for emphasizing or catching attention. If you format the whole description, they are pointless and can be harder to read.
The same applies for any other content apart text. Only use it if it's definitely required. Put a smiley or two, but no more, otherwise you can be seen as childish and immature.

A good description will/can have:
A short intro so as to why you've created the file or a little story about your file.
Explanation of what your file is meant for and how the file works (this applies especially much to lua scripts!)
Short instructions on installation, like setting up lua scripts or where to extract the contents of zip archive
Credits. This is not the case if you made everything in your file without the help from/works of others.
If it's the case that there are problems or bugs. Make sure to notify the users in advance to prevent them from causing them.
If there is something that you think the user will forget to do but it's really important, write a note about it!

A good description will NOT have:
Author's name (unless the file was created by someone else and you got permission to upload it)
Tons of unnecessary smileys/emoticons
Words like "please like" or similar
Words like "this is my first" or similar
DOs and DON'Ts (Rules) except for rare cases when some rules may be written. In most cases, rules are NOT NECESSARY because doing most of the things listed would be disregarding license laws as well as UnrealSoftware.de uploading guidelines.

I believe statement "a picture is worth a thousand words" is true. Good images will increase the quality of your upload A LOT. When uploading, please try to add at least 1 or 2 images. A file with no images is boring and not many people will pay attention to it. The same applies if your images show nothing out of ordinary, try to find what you think is interesting and screenshot exactly THAT.

If you're uploading a skin, it's best if you upload screenshots of:
drop image
hold image
buy image (if you made it)
a zoomed image to show the details of your skin
Possibly you can add a screenshot of the real weapon and/or player/zombie/whatever you made the skin of. It just has to be related.
You shouldn't upload images of:
Anything not related to file
Related to theme of file but still not related to the file
Same kind. Example. A drop image in blue background and the same drop image in grey background.

If you're uploading a map, the best would be if you added:
4 screenshots of you playing in it
a map export
If you do not know how to create a map export, read [GUIDE] How to create a map export in CS2D?

I do not advise to add screenshots from any other Counter-Strike versions as they don't show anything about the actual map.

If you're uploading a lua script, try to show some functions of your script and what the script does. If the script does some kind of process, it's best if you screenshot the whole process and the results.

When uploading, take note that you cannot upload more than 5 images so choose which images represent your file best.
Use a standard installation of cs2d to do the screenshots so people can see the "new stuff" instantly by taking a look at the screenshot instead of having to find out whether the changes are actually in the file or just a different mod.

The file is obviously the most important thing in your upload so it makes sense to make it as good as you can. There's no fool-proof way of creating an amazing file. You may look for ways or other tutorials on script/map/skin/sprite making but I'd like to note not the actual content but how you present/pack it:

Making the file
If you're making a lua script:
> Never overwrite the "server.lua" as some people execute multiple scripts with that file and overwriting it would make more work for everyone
> Code the script in a way that it can be added by adding a single dofile() to the server.lua
> Make sure your file is compatible with CS2D File Archive Restrictions as those that don't almost undoubtedly will be removed (and uploaders temporarily banned most likely)
If you're making a skin/sprite
> It is advised to use standard installation of CS2D (with default skins and sprites) so people could see the "new stuff" instantly by taking a look at the screenshots instead of having to find out whether the changes are actually in the file or just a different modification

Installation
I noticed that many people make file installation really inconvenient. For the sake of others, please put your files in cs2d folders. A typical upload will have a ZIP with contents as following:
EPICSKINS_BY_RAININGMAMMOTHS/image.png
EPICSKINS_BY_RAININGMAMMOTHS/anotherimage.png

Now lets say "image" is a new skin for cursor and "anotherimage" is a new skin for player.
Most ZIPs contain only that. To make it more convenient please do not put your files in any unnecessary folders. Instead the ZIP could contain :
gfx folder containing "image" and players folder (the players folder contains "anotherimage" and is in gfx folder)

This simplifies installation greatly. Yes, you know how to take a file, cut it and place it in another folder but some of the users do not know where certain files belong.
Making the archive
When archiving your files, make sure that your archive is in '.zip' format and select the best (if possible) compression option. This applies especially if your file contains images or sounds of greater sizes. Not everyone has fast internet.
Section
Make sure that you're uploading to the correct section. Otherwise, your file may be deleted.

Likes
Please understand that likes do not determine anything in this site. There's no benefits of having more likes than other users and will earn you nothing. Do not ask to like your files - it defeats their purpose. A "like" is used to show that a person enjoys your creation. Just that. If you see a person hasn't liked your file, the file may not be for his/her taste or maybe he's given reasons why. The reasons can be found in user comments.
Comments
A comment is a place where you get feedback on your file. If you see a comment that does not give feedback, ignore it. If it's off-topic or breaks the rules, it will be removed by moderators.
With that said, many people react negatively to "negative" comments. The comments aren't negative but most people get offended that the user did not appreciate his/her work. Sure, it's bitter, however, you get great information on why your file is subpar. Use it! Just think about it - you get a lot of people telling you what you did wrong. It's a great chance to improve your skills and techniques.
Do not flame at "negative" feedback. If another user agrees with the user that you're being offensive to, it's very likely that he'll also start disliking you or even your file. This results in flame wars, disrespect, and just generally negative atmosphere in the forum.

If it's not the first version of the file you can either :
Edit the previous file or
Delete the previous file and upload the latest version.

I do not recommend the latter case as all the likes, comments, suggestions and anything the people said that could help improve your file would be lost and basically, you'd have to do the same work (of uploading) twice.

When you update a file, I believe the most understandable way to show that the file is new is to mark it as new and update the screenshots with new. This does not mean that you do this every time you make a little change.