It has consistently worked for me:
- Disassemble everything around the zone of noise suspicion, namely remove the handlebar from the stem and separate every bolt and washer away from each other;
- Clean them with light solvent or oil (the idea is not to degrease, but more to remove dust/rust/dirt and to allow a residual layer of lubrication to remain);
- Apply light oil to a cloth and polish every contacting surface with it (middle of handlebar, stem clamp);
- Apply GREASE to the threads, but not elsewere;
- Reassemble with proper torque (tight, but not strangled). You'll find out that you get excelent tightening without too much torque when you use a recently cleaned and lubed bolt/washer pair).
My theory is that these creaking sounds are caused by micro-movements between dry, dust-affected, under-pressure surfaces. The cleaning and lubing do fine.
But... DONT DO IT if you're using carbon or any non-metal parts! Silicon might work better in this case (after the disassembly and cleaning, of course).
Hope this helps!