What I did when I was leash training Felony, was I would get a retractable leash and pull it out a ways, and let him drag it around the house with him.. I'd also use just a regular leash sometimes...
That way he got used to having a collar and a leash on, and having something tugging on it behind him.

Also, a while back when I was reading up on mastiffs, I remember reading that they shouldn't be walked on just a collar for some reason.. and that you should use a harness instead.