1. There WILL be lines. For everything. Plan for it. Deal with it. Do not whine about it.
2. If crowds make you nervous, have a plan to take breaks from the madness, or bring Valium. It can be very overwhelming.
3. Food is expensive, and outside food is not allowed. (Though I've never had my purse checked any time I've gone.)
4. Many celebrities will not just chat with you, you will be required to buy an autograph. Some will not chat at all, and will only do an autograph. Some will allow you to take a picture with them with your own camera, but will charge you for that, as well.
5. The especially popular celebrities will be difficult to get to see, whether by autograph, photo op, or attending their panel. If you're really determined to do these things for that celebrity, get in line as early as possible, plan to wait a long time, expect autographs and photo ops to be expensive, and don't be surprised if you still get turned away.
6. Photo ops are not a good way to chat with a celebrity. You will not have the time.
7. While Comic-Con is technically "family friendly", be aware that in the Artists area, there will be artists whose art may include barely clothed women.
8. If you're bringing children, be very well prepared. They will get tired and cranky, they will get overwhelmed, and any kid who's not capable of staying right by your side should be confined to a stroller or strapped into one of those harness-leash things.
9. Read up on the organization's website regarding the rules, especially if you plan to dress up.
10. Bring cash. A few places will take credit cards, but most will not, and it's cash only for autographs. There are ATMs if you need them, but you can be sure they come with a high transaction fee.
11. Do as much advance planning as possible. It will make your experience much better.
* Bring more cash than you think you need.
* Plan for longer lines than you think there will be.