Sometimes solving a simple creak is as easy as undoing a quick release wheel and chnaging the way the wheel sits in a drop out.
Cleaning the seatpost and seat tube collar has helped, just get your torch right when resetting. It is a good this to check that the post is not binding with frame every so often too. Apply a little grease to insure it doesn't bind.
Also reseting the headset. The top cap screw is used to snug up the races and bearings, this can only be done when the stem bolts are loose. Loosen, clean, snug it up (with top cap bolt), then tighten the stem to stiring tube. Threaded headset are a little different but just as easy to pop out clean, grease and tighten.