I recently acquired my first fixie, and all was well until my left crank arm started coming loose after around 20 miles. It was wobbling back and forth, so I walked the bike to my LBS and picked up the 8mm allen wrench needed to tighten it up. After tightening it with considerable force, the drivetrain gave me a lot more resistance than usual on the way home. I took the left crank arm off completely and the drivetrain was completely smooth again. As I screwed the left crank arm on again, it stayed smooth until the last half-turn or so, where I really had to apply some pressure with the allen wrench. At that point, the drivetrain starts to show a lot of resistance.
Am I tightening the crank arm down too hard? If I fail to tighten it down very hard (stopping before the drivetrain shows much resistance), the crank arm just gets loose within 10-20 miles to the point of wobbling uncomfortably.
The cranks are sakae signatures on a loose-bearing square taper bottom bracket.