Why does my bike crank creak and come loose? I recently purchased a Trek FX 7.2 and about 2 weeks after purchase I was riding and I noticed a creaking sound and shortly after the crank fell off. I brought bike back to where I purchased it and they put the crank back on and put the chain derailleur back on as it fell off. Great, no problems with the bike until about 1-2 weeks after where I was out riding and I noticed a creaking noise on the same spot, the left crank. As soon as this started I went right home and tightened the crank the creaking went away. I guess what I am asking is what can I do to the bike so that it does not creak and fall off every 2 weeks. I am a large person standing 6 foot 1 and about 270 pounds. Is the problem I have just a heavy man problem where I need to bring the allen key with me always and tighten it every 2 weeks or can something be done to avoid this entirely?
Square taper cranks are easily damaged if they a ridden loose. You may find that the only fix is to replace the crank. If after tightening to the correct torque they continue to loosen, they must be replaced. Over tightening, while tempting, is not the correct solution and will lead to maintenace problems down the track. (Essentially someone will have to remove the crank for some reason, it will keep coming loose after that and eventually be replaced. The process of fixing will take several attempts as the presumption will be the crank is in good condition when it is not) 

Take the bike back to the the shop. As the crank has come loose again, ask them to replace it. If they refuse, be prepared to take your bike back every two weeks until they install new ones. Do not "fix" the bike yourself. The LBS might claim it was something you did that caused the problem.