Thick tacky grease in the wheel bearings helps to add some resistance. Also get some inexpensive kevlar belted tires (no more than $12 each) and run them at the minimum air pressure on the sidewalls, or slightly lower. And finally, wear tight bluejeans and a loose shirt when riding.

ps, I forgot to mention that you can get up to around 6 watts resistance with a bottle dynamo to power headlights. Get a couple, one for each wheel.