Most of these suggestions come from http://www.apple.com/batteries/ipad.html, like FranceHopper mentioned. I typically start with his step #4, which can be found here: http://support.apple.com/kb/HT4211. In a nutshell:
1) Tap the HOME button to return to the Home screen, where you see all of your application icons.
2) Double-tap the (same) HOME button so the screen dims/grays out, and a row of apps appears at the bottom.
3) Swipe across those apps at the bottom to see how many are running in the background: each page will have 6-7 apps (depending on how you're holding the iPad), so 5 pages of 7 apps gives you 35 running apps.
4) Hold your finger over any one of the apps across the bottom until they begin to wiggle (3-4 seconds). They should all show a red '-' symbol on the top-left corner of each icon.
5) Hit the '-' for ALL applications
This does not delete your apps. It simply closes them. You will re-open them from the normal Home screen like you normally use to open the applications.
After that, try the other steps that FranceHopper mentioned above. Your biggest battery drain typically comes from anything accessing the internet through Wi-Fi or 3G (if you have that option), so closing those apps will make the most dramatic impact.