hi! I had that problem - there are a few ways to keep lucid.
1. every day, just look at your hands/watch and do RCs then, but don't think REALITYCHECK. when you are dreaming and think Reality Check your brain will take it as a cue to wake up I guess.
2. once you are lucid, don't try and stay lucid for as long as you can. instead, just say something to do with what you wish to do then let lucidity wear off. the dream will probably now follow that course.
good luck with lucid dreaming