Apart from not doing anything illegal (or at least openly advertising that you're doing anything illegal), there are basic rules of thumb to follow in any citizen/police encounter. For those who don't want to click through everything, here are the basic steps to follow if you encounter a police officer at Coachella or anywhere else:
1) Be calm, cool, and collected.
2) Be respectful, but firmly exercise your right to remain silent and to not answer questions you feel uncomfortable with.
3) Respectfully refuse searches if asked (note: this will probably not work well at the Coachella gate, since you are giving your authorizaton to be searched as a condition of entry).
4) Don't get tricked, and be careful of strangers. The police are allowed to lie and use subterfuge (as well documented in these and similar threads).
5) If you are being detained, continually ask if you are free to go.
6) Don't expose yourself (again, if you light up a joint in the middle of the polo fields, or start acting like the naked wizard, the constitution probably won't be able to help you much, no matter how lax law enforcement traditionally is at Coachella).
7) Don't EVER run.
8) Don't EVER touch a cop.
9) Report police misconduct and watch out for the rights of your fellow concert-goers (remember, all of those recording devices can be used to capture things other than buzz bands).