I've had the same plates for YEARS, and keep 'em every time I get a "new" car. Yes, they look a bit beat up, but I don't care. I know the plate number and don't feel like trying to memorize a new one.
There's a trick to learning them: do what the cops (?) and military do and use words for the letters. So if you're plate is CK4-11L you can think of it as Cathy Kitchen four eleven Larry, or whatever works best for you. My plate starts with letters that are also one of my aunt's initials, so that's how I remember it.