2008 was my first ZendCon experience and I'm happy to say that I've been going ever since. Here are some tips based on my experience at previous conferences. These tips are meant for newcomers, but may be useful to alumni as well.
Participate in the backchannel. Last year's conference attendees generated over 1,500 tweets using the #zendcon hashtag and reached the top 5 trending topics on Twitter three days in a row. Over 2,200 photos were posted to Flickr using the zendcon11 tag as people traveled to, attended, and departed from the conference. There were also many blog posts and the #zendcon IRC channel on freenode was quite active. You can participate by using the Twitterslurp of ZendCon 2011 or using tagnabit.net/zendcon (I'm the co-creator of tagnabit).
Attend at least one ZendCon UnCon session. The unconference is, "a community-driven event focused on interesting topics of the moment, and engages everyone at whatever level they can." No paid speakers, no ridged structure, just sessions of interest to the community by community members. Keith Casey did an awesome job organizing last year's unconference and I suspect this year's unconference will be just as interesting!
Attend a few sessions outside of your normal topics of interest. There were a few sessions I didn't go to last year that, retrospectively, I wish I had attended. One that comes to mind was Ben Ramsey's Distribution and Publication With Atom Web Services talk. At the time I didn't know a lot about the Atom Publishing Protocol but now it's a topic that I'm very interested in. You never know what you'll learn if you introduce yourself to some topics you're not currently familiar with.
Spend some time in the "Hallway Track." ZendCon isn't just about attending sessions by some of the most respected people in the PHP community, it's also about meeting many of these same people and others from the PHP community. The PHP community tends to be very friendly, so don't be shy! In fact, you will probably have already "met" some of these people in the backchannel. If you see someone who you've talked to on Twitter or in IRC, don't be afraid to say, "Hey, I know you from Twitter/IRC!"
While I was writing this blog post, Keith Casey published a great post with some dos and dont's for your first conference. Definitely check it out before packing for ZendCon!