Don't repeat the callsign of another station many times like you would on hf! Avid hf'ers sometimes don't realize that VHF/UHF REPEATERS are different, and that it's not necessary to do this on repeaters. Of course, you may repeat any call multiple times spaced a short time interval apart.
Don't use the word "BREAK" to interrupt a repeater conversation unless you have an emergency! While this term may seem expedient, it is not recommended for normal use. Instead you could simply insert your CALLSIGN to gain attention. If this doesn't work, try saying "HELLO...." or some other short insert.
Don't operate on the output frequency of a repeater unless you are conducting a path comparison test or other extenuating circumstances! While you may have the right to do this, it is not a courteous thing to do under normal circumstances.
Don't talk so long you time the repeater out! Anyone can be a "BUCKET MOUTH!" If you have something long to say, let up on the mic once in a while to let the repeater reset. It will also allow others to break if necessary.
Don't be guilty of OVER-ID'ing on repeaters! You may be very proud of your callsign, but others do not need to hear it every time you transmit. Remember also that the rules for ID'ing have changed and ONLY require that you give YOUR callsign when making an identification. This does not mean you cannot freely give the other stations callsigns, but please insert the ID's sparingly. It is worse to NOT give ID's when required, but also undesirable to OVER-ID.
Don't use the "DOUBLE-LETTER" expressions for legal ID's! If your callsign has a double letter in it, like AA5XX, refrain from saying "DOUBLE-A, 5, X X..... The FCC frowns on this practice but does not vigorously enforce this infraction. However, it has made this point clear in the past. You may insert anything you want to in between your legal ID's.
Don't use fancy PHONETICS when they're NOT necessary! Phonetics are designed to clarify calls and words in voice communications. If your reply is YES or NO, it is doubtful that these words need phonetics to describe them. Use common sense.