im not a judge, but my coach is actually an elite judge (which actually helps out a lot because i know a lot of my deductions) but anyways. heres some stuff
1. you are always judged from the beginning to the end of your routine. so this includes for example from your salute and walk onto the floor, and your salute and walk off of the floor (i don't know if thats all TRUE but thats what she says, so it can't hurt)
2. try to fix every little tenth you can possibly fix. bent knees, extra steps on the landings, flexed feet, ect.
3. SMILES ON FLOOR, if your music is kind of serious and it looks ridiculous to smile than obviously don't smile, but more often then not you can smile.
4. on floor eye contact with judges. I know its scary, but after your first time doing it you won't be afraid of it anymore. it makes you look extra confident and it gives some expression to your face
5. if your a high level gymnast then on bars hit your handstands...always.
6. for beam, work on your fluidity of your routine, no extra pauses, no weird dance poses, ect.
if i remember more i'll post it.