What is the best way to play the middle stages of a MTT when you have an average stack?

Question

Escpically when your M is around 15-18.

Answer 1

play tight out of position and be aggressive on the button and possibly the cutoff. If you have a situation where you know you are ahead, push ... if you lose you lose, but with an M of 15 you need to push when you have an edge
With an M of 15 most of your preflop raises should be about 3X the big blind or so. I would still be pretty reluctant to enter a pot if others have entered it.
When the M goes below 10, you can push all in when nobody has entered a pot with lesser cards than before.

Answer 2

Purchase all three volumes of Harrington on Hold'em. In these books Dan Harrington provides detailed insight into exactly how you play the middle stage of a tournament at various M levels.

These books are the best books I've ever read.