Are you trying to brighten the eInk side or the LCD side?

If it is the eInk side, I'm afraid there isn't anything you can do. Maybe someone can correct me if I'm wrong, though.
As for the LCD side, when you're on the home screen, press and hold an "open" area until the menu pops up. Touch widgets, and then put the power widget on your home screen. This has a button to cycle your LCD through 3 or 4 different brightness levels. I find that very useful.

Hope that helps!