Seidio's normal-size extended Palm Pre battery adds about 20% juice
I actually bought and use a very similar battery from Seidio for my T-Mobile G1 and I’ve been pretty happy with it (full review here). The Pre version is rated at 1350mAh and promises about 20% extra capacity, which should add a nice bump to your daily usage.
Innocell 1350 Extended Life Battery [Seidio via SlashGear]