Open Activity (Shift+Apple+A), and look at your running processes. Find the largest ones, and shut them off if they aren't vital.
If you have any questions about running processes, google the process name to find out if you can shut it off or leave it running.
I had some issues when I first got into Lion -- I had about 15 processes / applications running in my user/startup actions folder. Killing these greatly improved my boot time (I don't miss them, but I run them from Spotlight if I need them).