It's been a long time since I bought something solely for the purposes of improving my wargaming game. Wargames of the map and counter variety are largely self contained, so why would you need to buy anything extra?
These days I've been buying more pocket games, small and compact wargames that can be played in an hour and are sold in zip-lock bags. This leads to less games played on rigid cardboard maps and more games played on folded paper maps, which can be a real hassle. It doesn't look so bad when you just have two or three counters on the map, as in this illustrative photo, but once your stacks reach seven or eight counters, and are spread all over that continental divide, the slippage can drive a man crazy.
To fix this problem, I finally went out and bought a cheap piece of clear plexiglass sheeting. An 18-inch by 24-inch sheet costs about ten bucks, and can be scored and cut in half using a boxcutter. That gives you two sheets, one for use at home and one for use during those lunch games at the office, that will fully cover a standard ledger sized map (11- by 17-inch map).
After scoring and peeling off the label, it forces the map to lay flat and provides a much cleaner and easier playing surface. It's so nice I can't believe it took me this long to get around to it. Look how much more convenient it is, no tiltage, no slippage, it's great! This leaves the mind free to focus on the rules, the tactics, and the opponent.