Dancesafe.org has a pretty useful guide:
http://dancesafe.org/content/story/win-summer-festivals
Before it happens, because it always comes up in these threads:
No advice or requests for ideas on getting drugs into a festival.
I encourage you to research the venues and event websites you are going to, if you haven't already, so you know what expect in terms of layout, location, and security. Certain events don't allow in things that seem unrelated to safety and drug use- better to find out beforehand. It's also a good idea to know the laws in the area you'll be in as far as public intoxication, drug searches etc. Be prepared to remove your shoes and have a pat down, including around your bra area by a female security guard (yeah, they're allowed to get pretty close). Entrances can sometimes be hard to find because the entire area is swamped with people, but sometimes if you check out the map in detail you can find some lesser-known entrances that may be less crowded than others. Expect to wait for a while regardless, as the staff at these events tend to only do these jobs a few times a year.
As far as enjoying the events on a chemical level, it's never a good idea to have MDMA be your go-to drug for multiple days (or even weeks) at a time. Though you may break the 30 day rule and be fine, I'd recommend not doing this to the extent that you're intending to roll 3 days in a row. Enjoy other things- if you are 21 and like alcohol, take advantage of being able to consume that openly and legally at the event one day perhaps. Weed, K, and various psychedelics can be really fun too. I would plan which day at each festival will be the best to use MDMA at.
It sounds like you personally will already have your MDMA covered, but for those hoping to acquire drug inside a festival, I strongly advise not doing this under any circumstances. In general, you can stay out of trouble at these events and be as high as you'd like so long as you don't get involved in the buying or selling of drugs. Typically, I find that at events I'm asked if I "have any pills babe?" every 10 feet, and I usually am certain that the guy asking me is undercover. Don't talk to anyone trying to buy pills or anything from you, it's just not worth it.