1. Make sure the feature you're suggesting isn't already in.
You can usually check by browsing the Wiki and looking for whatever you're after. If it's not there, it's a good idea to have a quick scan through the Changelog (Use Ctrl+F to search using keywords that apply to the feature you want. For example if you wanted to know if GZDoom supported custom damage types, use a word like "damage" or "damagetype" in the search box) to see if it hasn't been recently added (meaning it will be in the next GZDoom version). If you haven't found anything, ask about it in the Editing forum, just in case.
2. Give an example where your suggestion would be useful.
Unless it is really obvious how your suggestion could be handy, please include an example where you could use it.
A suggestion such as "Players able to be viewed as skyboxes" with no description or example on why one would want to make a player a skybox, would be a bad example of a feature request. Unless you can come up with a good example why authors would use the feature you're requesting it probably won't get added. Things such as "Because it would look cool!" etc are not really acceptable.
3. Don't suggest huge changes.
By "huge," we mean things like "3D Models in Software Renderer", "Video rendering support", "In-game Internet Browser", or "Better Netcode." Major changes which require a lot of work should be up to the developers, and will probably end up with [Later] or [No] tags.
4. Don't re-suggest things you have suggested in the past.
If your suggestion gets No'd within good reason, don't post the same suggestion again. By all means try to work out and debate around the refusal in the Closed Feature Suggestions forum, even if it means bumping an old topic. More importantly don't get angry/upset and begin flaming if your feature wasn't added!
5. Want to help clean things up? Please do so responsibly.
Once in a while, someone will have suggested a feature that is already in, or you might find an old thread suggests the same feature as another, newer thread. To help the devs clear out the list, please bump the thread and mention that the feature should be closed. Please don't use the Report Post button, as the devs do not often check the Reported Posts, and this also clogs up the Reported list for the moderators who are not necessarily devs.
Closed request reason reference:
[No] - The request was not accepted. Read the thread to find out the reason.
[Added]/[Done] - The request was added and will be in the next version of GZDoom (or a development version)
[Already In]/[Not Needed] - What you requested is already in GZDoom and/or there is another way to do what you requested.
[Later] - Will be reconsidered at a later date (Maybe).
[DIY] - GZDoom already has what is necessary to handle the feature you are suggesting, and implementation is your job, not the developer's.