Learn them, forget them, then learn them again a few times over. Eventually they'll sink in. I keep my air law book in the flight bag, and read over a list of things I read every time I fly, simply because I know there are certain things like signal squares and light signals I'm prone to forgetting.
My impression is that even CPLs and flight instructors are prone to forgetting certain things - this being one of them. The only time I've seen a flashing red light, the pilot I was with promptly stopped the aircraft, whereas in fact it means 'move clear of the runway'. Luckily, we were already clear of the runway. Perhaps the tower had the signals mixed up - why else would they have shone a flashing red light at us when we were already clearly off the runway?