I have stray quilt blocks, that are a result of testing to see if i want to do an entire quilt, trying out different techniques, etc... So this is what I do.
1. Finish the block with borders.
2. Quilt or not.
3. Cut a piece of cardboard a little smaller than the block.
4. Staple some batting to the cardboard that is large enough to fold over the back at least 3/8" (if not quilted).
5. Pierce a hole the size of a thumbtack in the back (not the flathead, but the pushpin type).
6. Lay the block on the top of the cardboard piece, fold the edges of the block to the back and tape down (taping allows me to reclaim the block if i so desire).
7. Place the head of the thumbtack in the hole created and mount it on the wall (this allows you to avoid making a nail hole in the wall, which is generally larger than a thumbtack). Also, the thumbtack does not protrude, the cardboard rests lightly on the thumbtack and is buried by the layer of batting.
:) Hope this inspires you. :)