The nucleotide mnemonic I use is:
GACT. I like it because it's goofy sounding, so it's easy to remember. The purines are before the pyrimidines (just like purine comes first in the dictionary). you can rearrange it like so:
to remember that G binds to C and A binds to T. GC bonds are stronger than AT bonds. So that pair comes first.
for RNA, it becomes GACU (pronounced: "Gackoo"). Everything about GACT is also true about GACU. If you can remember both, you will easily be able to remember that U replaces T in RNA.