The biggest mistake I see people doing when they start out with powercleans, front squats, or any other clean, is that the elbows are in the wrong position when the bar is racked. Your elbows should be pointing straight ahead. Also, as someone else mentioned, moving forward during the lift to catch the bar is very bad. One thing I couldnt believe I saw was someone doing cleans with straps. You need a gym that allows chalk or at least hide the chalk in your gym bag and clean up the mess when your done.