Before going to sleep at night, repeat the following to yourself: "I will wake up at 6.00 am" (or whatever time) repeat it 10 times, and try to really mean it. Focus on the phrase and nothing else. Say it out loud, don't just think about it. Trust me, it works better. After repeating it 10 times, simply relax ans go to sleep. This has worked for me pretty much all my life, almost every time. Good luck.