I use an attachment like Mordy's except it attaches to the back wheel.
To start out with I would hook his harness up to the attachment and then also use our gentle leader with a leash attached as Gunnar liked lunge at other dogs in the beginning as well. Now I use the attachment and his regular buckle collar with a leash just in case something goes wrong with the attachment, I still have a hold of him.
When we would intially see another dog I would just speed up, he had to worry about keeping pace with me which helped him learn to ignore other dogs while we were riding, he does exceptionally well now, could care less about other dogs, he's just busy moving forward!
Gunnar weighs 84 lbs and he has never, even in the beginning, managed to pull me over on my bike.
Keep in mind if you do decide to bike with your dog you need to build up their pads slowly, can't go for two miles the first day, just a few blocks and then a few more. It is also best to not ride after the pavement is wet, can cause the pads to become soft and they are more likely to injure them. I also highly encourage you to get a reflective harness or "jacket" so your dog is more visable.
We have so much fun, on occasion he will wear his backpack too and will help me bring home some light groceries. It really seems to give him a job to do when we are riding.