Put your business cards behind your badge, so you can easily trade with people. When you exchange cards, put their card in your badge. When you run out of cards, take all the cards you collected out, and put a fresh batch of your cards in.

Money, ID and some credit cards are much thinner/smaller than a wallet.

Don't forget to fill out your conference evaluation forms! Take them with you and fill them out as you go, as comments come to mind. Then at the end, there's very little to fill out (just the overall stuff) and you're done!

Label your power brick, and label any connectors you have too (I put an address sticker on my Mac VGA connector).

Bring a water bottle and DRINK WATER.

Where to get water to fill your bottle.

Take a USB stick, just in case. If you're a presenter, put your presentation on a USB stick, and also upload it to the web BEFORE your talk so when people ask the inevitable "can we get the slides?" you can give them a URL right away.

Try to use as low profile a bag as possible. I use a backpack if there's a lot of walking, but for a conference with just a few rooms close to each other, I use a canvas bag (my 15" laptop fits in it).

A Tide pen - detergent with a scrubber tip. Just in case you get messy.

EAT BREAKFAST. And bring everything with you to breakfast so you don't have to go back to your room before going to the conference.