Barbed wire. Lots of it, with the odd AP minefield in the openings. Especially useful if you are facing lots of inexperienced infantry. AT mines and roadblocks can serve to channel the enemy but in larger maps you'll need to many for them to be cost effective. I never seem to put my bunkers in the right place, which is a shame because I've been on the receiving end of effective bunkers more times than I can count (1, 2, 3....duh).