Particularly if only one crank creaks, retorque its mounting bolt.

If you have a traditional open-bearing BB, inspect the spindle and cups for gouges, and try replacing the ball bearings themselves. If you have a cartridge-style BB, be sure to grease the threads of the retainers.

Readjust your rear axle bearing cones to eliminate the slop. (See an earlier thread regarding controversial proper adjustment techniques for QR wheels. I still favor leaving a small amount of slop, on the theory that clamping the QR skewer will force the cones to move minutely toward one another.)