A lot of good information posted already.
I fly in from So Cal every year, but have learned to go no later than the 2nd weekend. By the 3rd weekend, kids are out of school and the park is slammed.
As mentioned, your best bet is to either do the Stay and Scream, or get to the park entrance NO LATER THAN 6:00. Get through security and wai in the holding area right before the gates. Once you get in, head straight ahead to go to the SoundStage houses. Do NOT stop to even look at a scarezone. Do NOT stop and get a locker.
Go to each of the SS houses - the queues are horrifically long (plan 5 minutes minimum walking from exit of one to entrance of the next).
NOTE: EVERYONE goes to the SS houses first, so if the park promises to be especially full, you might want to bypass the SS houses and rush to the back of the park (Disaster/Amity) and do those first.
For the first approx 20 minutes of opening the gates, you will have free-reign, but after 30 minutes, the wait times will hit 15 minutes. After a hour, forget about it. They will stay that was all night - but will back off between 1AM and 2AM.
Another word of advice - always be aware of when B&T show lets out, and AVOID houses in that area for at least 45 minutes after the show.