Two things...
First - try to follow the examples of other mod writers. If you're including a new php file, make sure all the actions are defined in index.php, because this saves bandwidth and is a lot faster.
Second - make a note in the mod file that it is covered by the Terms of Service. This may not seem important, but we have had issues recently with people stealing mods for use with other forums. I don't want to go into detail, so just make a note that the mod is covered by the Terms of Service to help prevent mod theft.
Third - test the mod thoroughly, and make sure everything works like it should. I try to do this, but since I often "mod in realtime" (make changes to files right on the server via
net2ftp.com) and then try to create a seperate mod file based on the changes I've made elsewhere, I often miss a lot of bugs.
Other than that, have fun, and be creative. If you run into problems, feel free to ask for help. I'm sure a lot of developers will gladly give you advice and help you out.