Ya, if you don't have a regular sleep pattern, telling yourself to wake up at certain times does not tend to work, because your biological clock is not adjusted enough to just do this.

At-least, I know personally, I haven't had a regular sleep schedule in years, and I never wake up naturally at any time. I'm now dependent on an alarm clock. Sucks really, but I have tried telling myself to wake up at certain times, with fair confidence, and it just doesn't work consistantly.

Idea for you:
If you don't wake up naturally several times a night, set an alarm which doesn't require you to turn it off. In other words, something that will wake you up just enough, but not require to you move to turn it off, thus it self terminates.
For instance, I have a laptop which i can schedule "events" on. So i could have my laptop play a certain sound for like 20seconds, enough to wake me up, then that sound would end, and i could could do this technique.

Goodluck