Get a set of rollers. Learning to balance on rollers is quite a challenge, but they do wonders for improving your balance and pedal stroke. After using rollers for one hour, take a ride outside on your bike, you will notice a difference.

Riding rollers can give you an intense workout, and you can use them to ride indoors on rainy days. I set my rollers up in front of the television and watch movies. Old Martial Arts movies are the best, but Tour videos and such will work.