It's been pain since I started cleaning teeth properly.
Use a small headed toothbrush - I have a similar problem to yours, and I use two brushes to clean my teeth, one is the smallest possible standard headed brush, but the other has a very small head, and that's the one I employ when doing right at the back of the mouth. Actually, its a child's brush, because I couldn't find a small enough head in the adult range in the store at the time. Corsyodyl make a small headed, oval shaped brush, and that one's okay, Amazon sells them in soft or medium. The alternative is an electric toothbrush with a round headed brush, such as most of the Oral B range has, but if the interior oral cavity is small (as is mine) I find it quite difficult to manouevre a clunky electric handle and stem within the mouth to cover all surfaces.