Best Match is a total mystery to everyone (probably even to ebay themselves)but one thing I have noticed is the more you sell and the higher your sell-through rate, the higher placement you'll have.
A friend of mine who is also a competitor uses Auctiva without enabling the gallery photos, NO free shipping...but she is always placed highly in best match. BUT she has 3 or 4 times the amount of listings that I have, so I am guessing that is the reason. More items, more sales, higher placement.
Again, just my own observation