The diaper bag dictionary flash cardss are one fun tool. These are great because they're durable and on a ring so they're easy to carry around. It's also easy to break the deck apart and leave a few on the kitchen window sill, a few in the car, even a few actually in the diaper bag. That way, whenever you have a few extra seconds you can quiz yourself or learn something new.
The ASL Glossary DVD is also really nice because it includes a description and mnemonic for each sign instead of just a picture/video of what it looks like. It's easy to look up one specific sign you're looking to add to your vocabulary or browse through several as a refresher.
Practice with yourself in the mirror or at a red light. It sounds silly, but it really helps you to internalize how each sign feels.
Practice with your partner. Try to incorporate a new sign into conversation each week. BONUS: You can start talking to each other across the room or without someone else hearing!