The problem with using alarms is that it shocks you awake. This causes you to become too aware and unable to slip back into the necessary state needed to DEILD. Some people have success with this though, you may as well try.

What exactly is your mantra? Remember to keep it short and simple, positive and as if you already do it e.g. "I remain still when I awake".

Another useful thing to do is to visualise waking up from a dream and not moving, then transitioning into a lucid dream while repeating your mantra. Do this until you fall asleep, or if it keeps you awake do it a few times and then leave it.

Remember that there is no perfect method or secret to lucid dreaming, it requires patience, persistence and confidence in order to get good at it. Give it some time and don't beat yourself up when you fail, you'll get it eventually

Here are some good DEILD guides you should take a look at if you haven't already:

Yuppie's DEILD Guide

Dutchraptor's DEILD Guide

Aaaaand just to keep your spirits up:

Mancon's Guide to Staying Motivated

Good luck