you can get specially made seats for the bikes, or you can make them fairly easy (especially if your bike has a luggage rack) the other method is to have the passenger have a carrier on and puppy rides in the carrier.
some dogs LOVE it, some never get used to it.
rosie will be having her first ride in the spring, shell be riding in a carrier with my mum for now, but im designing a seat for her that will attatch securly to the passenger seat on the bike!
if your gonna bike with your dogs PLEASE invest in the time needed to get them used to doggles...
there are ALOT of biker dogs out there lol.