I am generally against the idea of using mnemonics.. but this particular one is a life saver for me.. though stupid :D
Think of a chewing GUM, think of the times when it was stuck to your fingers..and you struggled to get rid of it.
That's how valgum..when things stick to each other.
No more worries as to what is genu valgum and genu varus.
Additional mnemonic by Subbiah: After K is L, by alphabetical order. Hence Knock knees is vaLgum.