1.Not working the muscle,I can't stress this enough when you are doing your set your mind should be focusing on that muscle squeezing and contracting the whole way through.
2.Ego lifting-when you hit the gym tune out everyone respectfully,You have nothing to prove to anyone.
3.Staying Consistent-Set realistic goals like 1,2,3 year goals because we all like to think we will have our dream bodies in 3 months LOL which invariably leads to disappointment.... it is a lifelong commitment so enjoy the ride =)
4.Doing the same routine day in and day out-MIX IT UP use different rep schemes or techniques such as rest pause,drop sets,giant sets.total failure etc etc rotate them into your routine have fun