...as far as I know, you don't decide. I believe it's Jon that does that (unless the mods have a secret meeting to discuss it).
I've let my brother pick one or two way back near the beginning when he was helping out, but now that the web site is entirely my baby, it's up to me.
Often I try to relate the choices to current events, such as the releases of related games (or a couple times, related movies), or the season or holidays (Castlevania games in October is the most obvious). Sometimes the frequency of past MotMs sway my decision, because I don't want too many games from the same console, especially in a row, or I don't want to keep honouring the same people over and over again - though the latter is hard when there are some core contributors that will get multiple MotMs just because they've done a lot.
I had thought about democratizing the "Maps of the Month" choice, but unless I actually find time to do a revamp (there's that dreaded word again), that's not happening. And even if I did have the time, there's a few logistical concerns and I may end up wanting to make the decision myself anyway - a plain vote for any game is just going to lead to all the popular games being MotMs right away, and lesser-known games will probably never be recognized. Or it might be skewed since the regulars would end up having more of a say, and they'd vote for their own, and then all we'd see are Will Mallia's maps for the next five years. (I'm kidding, but Will does contribute a lot.) We could limit the choices to maps contributed in the last month, but then people might deliberately stagger or delay their releases, and I don't want that to happen.
