I think the biggest mistake in stillhunting is a combination of moving too fast and not taking into account the wind direction. The ground litter can be a problem also. If you are stillhunting on dry leaves then you just as well go home and try another day. It's best to try on wet leaves or fresh snow. The rule of thumb I use is, if I hear my footsteps then the deer can too. If you are covering more than 100 yards in an hour then you're moving too fast!
I have had my greatest luck stillhunting up creek beds. I'm talking water about 2"-4" deep with a solid clay bed. You can place your foot slowly into this and not make a sound. I shot a nice 6 pt. at 4 yards one time stillhunting in this manner. I didn't sneak up on him but I placed myself in the middle of the bedding area and spotted him about 40 yards away. He turned and walked straight to the edge of the creek and I was at full draw waiting. If it wasn't for the water in the creek, I couldn't have been in this position.
It's best to still hunt on windy days also. This helps to muffle any sound. If you crack a stick, then wait 5-10 minutes before proceeding. Sometimes a deer will hear the stick crack and if you are not moving will calm down. They seem to be extra alert for that second stick crack though, so you need to be careful from there on.
Probably the most important tool in stillhunting is a pair of binoculars. I can't count how many times I have spotted something that looks like a part of a deer and through the binocs have found it to be a stick or leaf. Sometimes though, you may spot a white patch that is the inside of an ear or the under belly. I've spotted the black nose alot and the horizontal line of the back in a mostly vertical lined woods. Without the binoculars I may have blown it. Hope this helps! Good luck!