Hold out your left hand and write "left" in the air using it as the L. After a few years of doing this (maybe two), this fixed the problem for me 99.99% of the time.
I actually have a rather big mark on my left hand and my grandmother taught me to differentiate left and right with that mark on my hand, which I still use as I sometimes get confused. Weird but true.
Once I got tattoos on my forearms it made figure out left and right so much easier.
When I was a kid I once wrote L and R on my forearms with a marker both for remembering which was which and to learn their name in english. I think it worked, because I always think of the corresponding forearm when I have to tell left from right.
I have to refer to the hand I hold a mouse in, before that I used to imagine holding an n64 controller.