2003 Dodge Neon P0071 Code

I was just wondering how to properly fix this code. The P0071 code has shown up before on my wife's car and it was supposedly fixed by the mechanic. I just need to know how to properly fix this so it doesn't happen again. If it is the gas cap, we have replaced it before when the code came up and it didn't make a difference. She also tightens the cap more than three turns, so that isn't it either.