The city of choice from where you can reach Bandiagara is Sévaré or the nearby more touristic Mopti.
There are shared Taxis or some Minibusses making the way from Sévaré to Bandiagara. Ask at the "busstation" in Sévaré. To go to Sévaré comming from Mopti is no problem, as there is relativly dense traffic on this route. You will be able to find a place in a shared taxi or to hitchhike.
They were about building a new, paved road from Sévaré to the south (in 2001). As long this road is not finished, it will be a unconfortable ride on unpaved roads to Bandiagara ;-)hb
Below the escarpment tourists either walk and have a backup 4-wheel drive, or they use a donkey cart. I like the donkey cart because the 4-wheel drive is expensive and unnecessary. At the foot of the escarpment, the guide or you can make arrangements for a donkey cart to put your gear. If you buy souvenirs, then you'll need the cart for sure. In our trip, my wife rode on the donkey cart a good deal of the time, and in a way I wish I had to save my energy. I feel sick from overexertion in the hot sun, but the donkey cart and his driver had already left to do another run just after we arrived at the encampment.
Thanks to cheap Japanese/Chinese imports, the world has gotten a bit smaller for some Africans. At most came site you can find a local with one who will give you are ride for a few bucks. I got on one because I had to catch my ride back to Bandiagara pronto after a long lunch. No helmets here but the sand that you will be riding on is soft so if your driver happens to lay it down.