Some other things:
Post which mission you are running if your problem only occurs in a specific mission. This will help us isolate the problem and try to simulate it in a test mission.
If you're not a coder, please don't tell us which part of the code you think caused the bug. Sometimes we may listen to it and be sent on a wild goose chase.
Sometimes it may be helpful to figure out when a bug was present. If possible, try it in retail if your bug doesn't require new SEXPs. An archive of old exes can be found here:
http://fs2source.warpcore.org/exes/latest/. However, the feature you may be using may be newer than some builds there, so start moving back in 2 week increments and start narrowing it down if possible.
Feature requests in Mantis will be closed. Please spam the forums

Choose a category that best suits your problem:
HT&L: should not be used.
DirectX: graphics bugs that appear only in DX
OpenGL: graphics bugs that appear only in OpenGL
graphics: graphics bugs that appear regardless of API
FRED: non-graphical related FRED problems.
FRED grahpics: graphical related FRED problems. This should only be used when there are rendering problems, user interface errors should go under "FRED"
gameplay: Any general bug that occurs while in mission. Usually AI or balance problems.
user interface: non-graphics related bugs for the UI screens out of mission - barracks, pilot files, tech room, lab, breifing, etc.