Note: My experience is with Creation only. I can't say how other companies run their conventions or if any of these guidelines would work for them. I believe most of this would work for any convention.
General Advice: (These are in no particular order, btw.)
1 - Be a sponge. Soak up every moment.
2 - Make time to meet people and make new friends. You'll find that a lot of people at Cons have never heard of LJ (shocking, I know). Half the fun of a convention is meeting up with your LJ buddies, but don't forget to strike up conversations with new people, too.
3 - Interact with the guests and remember they're just people, too. Treat them the way you'd like to be treated. Be respectful.
4 - Don't ask embarrassing questions of the guests. Leave your politics and causes at home. I can guarantee you that neither the guests nor the fans will be happy with you if you decide to ask or bring up something awkward. Booing and the swift end of a panel often results.
5 - Hang out in the main room as much as possible. Everything happens there.
6 - Bring your patience because there's lots of long lines and waiting.
7 - Keep your eyes and your ears open. You never know what you'll hear or who you'll see. I've seen a lot of con guests pass by while standing in photo-op lines.
8 - Wear comfortable, nice clothes. There's no need to be overly formal at these things (though some people do).
9 - Bring a bottle of water. Usually there's water available in the main room, but it's better to be prepared.
10 - Bring a mirror, brush/comb, makeup, etc to prep for the photo-ops. It's handy to have all this with you so you can tidy up while waiting in line.
11 - Have a camera? Make sure your batteries are charged and memory cards have a lot of free space. Yes, bring extras of both. You can easily end up taking hundreds of photos over the weekend. Make sure to do a backup of all the day's pics if you can.
12 - Be respectful, whether it be to the guests or your fellow fans.
13 - Grab a bite to eat whenever you can. I usually pack some snack bars or little lunch kits in my bag, just in case there's no time to have lunch or dinner somewhere close by.
14 - Have a printout of the convention schedule. Creation usually has these readily available when you register. Refer to it often because they don't always announce when things are happening.
15 - Be prepared for the convention to get off schedule from time to time. Guests are sometimes late, things run over, and sometimes they have to re-adjust the times of things. Go with the flow.
16 - Wear comfortable shoes. You'll be standing in lines for registration, photo-ops, and autographs and it can take a long time.
17 - Shower frequently. Lots of bodies in rooms together means a lot of closeness. I bring some facial wet-wipes with me to freshen up a bit (on my face, neck, arms, whatever) if I've been running around from event to event and have gotten hot.
18 - Be nice to the volunteers. They're usually fans themselves and working long hours. Sometimes they're lucky enough to get to get close to the guests and it's cool to talk to them and occasionally hear some neat stories.
19 - If there's a problem, let someone with Creation know. They'll try their best to find a solution. Last year I did this when I missed getting a photo-op. There were a few others who missed out as well and when I approached Gary (one of Creation's founders) about it he assured me that they would fit us in and they did.
20 - Be prepared for the post-con depression and beware of post-con crud. The day after, with the trek home, then getting back to "normal" life can be rough. Give yourself a couple of days to recover if you can. Get as much sleep as possible and maybe bulk up on your vitamins during and after the con. A lot of people get sick after a convention. It's a draining experience, so pace yourself.
21 - If you have a camera, take lots of pictures. Take pics of things you wouldn't think of at the time as important: fellow fans (especially your seat buddies), the banners in the main room (do this the first day because they then auction them off the other days), the Sunday Breakfast room, your hotel room, the convention hotel. These all might be things you want to remember years from now and you'll be glad you captured them when you had a chance.
22 - Wear layers. Convention rooms can be hot or downright frigid. Best to be prepared either way.
23 - Go to the vendor's room early in the weekend. The glossy 8x10 pics are usually in abundance until the last day, but other items like posters, jewelry, keychains, etc. can sell out earlier.
24 - Have FUN! :D
* Creation Gold Tickets always sell out fast, in fact many of them are bought up by current convention goers as they buy upgrades for their seats for the next year's con.
* Creation charges $7.75 shipping & handling fee for all their tickets. The photo-op tickets for Jensen, Jared, J2 sandwich, and sometimes (but not usually) Misha sell out prior to the convention. However, you can save yourself some money if you're planning on getting a lot of photo-ops by waiting until you're at the convention to purchase. This way you pay the straight price without Creation's added fee.
* Same applies to the Autograph tickets at Creation cons. Outside of tickets for Jensen & Jared, you can easily wait & buy these at the convention.
* Depending on who the photo-op is with the lines can be extremely long. At first it'll seem like nothing is happening; you're just standing and waiting. Once the line starts moving though, it doesn't take long before you're in the room.
* For Creation Cons you put your bag/purse/belongings on a table and they're guarded. Don't forget to take off your ticket lanyard, too. You'll pick it all up on your way out of the room.
* I really enjoy watching people getting their photo-ops ahead of me. Gives you a good idea of the mood of the guests, too. Usually there's music playing and the atmosphere is fun and relaxed.
* I won't bother to say "don't be nervous" because even I've found that impossible. Just try to remember they're human, too and this is your moment to create a memory for yourself. Creative poses are fun, but remember to be respectful. Jensen & Jared are usually pretty game for whatever the fans come up with.
* If you're doing poses with or without props, make sure you know what you want done.
* You won't have much time to interact with the guest, but you can say "Hi" and a few other quick words before the picture is taken.
* Don't forget to smile and thank them after the pic is snapped. They really appreciate it and it's more time to talk to them. :)
* With Creation, they usually don't call too many rows up at a time, but the waiting can still be long once you're standing in line.
* Have your pics, etc. ready to be signed.
* Autographs are usually where you get a little more face time with the guests. Have something in mind to say to them or talk about briefly.
* Don't linger in front of the guest too long. Remember there's a lot of fans behind you.
* It's a good idea to have something to put your autographed pics, etc. in to keep them protected.
* Always use the escalators. The elevators take FOREVER.
* The Green Room (aka Gulf Islands rooms) is by the escalators on the floor above the Grand Ballroom. Sometimes you can see the guests going to and from this area.
* Bar One, in the Lobby of the North Tower is also a popular hangout for guests. Again, right by the escalators, so you can't miss it.
* Don't forget to check out the St. Andrews-Wesley church right across the street from the convention hotel. It was used for "Salvation" & "Houses Of The Holy" among lots of other productions like Dark Angel.
If anyone has other pieces of advice to volunteer about the convention-going experience...Go for it!
Edit: Be sure to check out the comments for more advice. ;)