We get a lot of folks in here at Redstone that are annoyed by creaky bikes. I'm not going to write an entire tech article about it - there's lots of websites out there that explain tech stuff better than I - but here's a few things to look for.
9 times out of 10, creaks are caused by dirt. A clean bike is a happy bike. Rarely is something creaking because it needs to be lubed. Generally speaking, the opposite is true. Follow this logic - most bikes have sealed bearings in suspension pivots, etc. The lube is in the bearings, not the pivot bolts. Bolts and metal/metal interfaces must be clean and tight for the thing to work properly. Lube attracts dirt. Dirt makes things creak. If you put lube on a suspension pivot, it will be a dirt magnet. Remember, dirt makes things creak. If you can isolate a creak to a suspension pivot or link, take it apart, clean it thoroughly, and put it back together dry.
Here's a few others. This bike was creaky under power and the bb has been pulled relatively recently. When I decreak a drivetrain, I almost always start by pulling the bottom bracket, cleaning, regreasing (yes, use LOTS of grease on the bb threads) and re-assembling. If it still creaks, keep exploring. Some out of the stand testing revealed that a creak was coming somewhere from the top of the bike. We pulled the post out of the frame and pulled the seat collar off to find lots of gritty sand particles. We cleaned everything up, including the binder bolt, and put 'er all back together (yes, grease your seatpost, too). The creak was less, but still there. Look at what we found on the saddle clamp...
Once we got that back together, the creaking seems to have been silenced. But there was still creaking in the front end. Simple disassembly of the stem/bar interface showed this:
Putting it back together, again, made things better but not 100%. From that point, we pulled the stem and headset spacers, even took the fork out. EVERYTHING had this gritty sand in it so we pulled the hs bearings from their cups and cleaned absolutely everything spotless. Guess what? Creaks are gone!
Creaks can be hard to diagnose and even harder to solve but hopefully some of this will help you on your way to decreaking your bike.