Email or call your local Warped Tour venue a few days before the show to see if the crew needs any help setting up or unloading. All those stages and tents have to be set up somehow. If you can get this gig, you'll usually obtain a backstage pass for lending a hand for free. If the venue offers to pay you though, try to negotiate for a pass instead.
The day of the show, arrive at the venue anywhere from 2 to 4 hours before showtime. Find the setup crew and ask if any help is needed--sometimes crew members back out at the last minute, so they might still need help. Ask for a backstage pass in return for your help. If there is an extra pass for you to have, the crew will make sure to get it to you.
If you have no luck with the crew, survey the entire area before you head inside. Oftentimes, there will be a fence around the area where the concert is held. Find out where the two main stages are being set up, and most importantly, find the buses and trailers that belong to the bands. Sometimes the buses are behind the main stage, sometimes in another lot altogether.
Walk around the area with the buses and trailers and see if you can spot anyone hanging out. If you see someone who looks like a band member or crew, you can see if he has an extra backstage pass. Each band is given a number of passes to use however they want--if you catch a band member early enough, she might have some extras.
The last option, and probably the easiest, is to make friends with a band once you're inside the venue. Around 40 to 60 bands play each date of the Warped Tour, and with each band having a tent, it's easy to meet them. Look for the lesser known bands you've heard of and strike up a conversation. Make yourself look cool and the band might give you a backstage pass or sneak you backstage. If you've got a band member with you, it doesn't matter if you don't have a pass.