i can't think of any mnemonic devices to use for that, but the way i think of it, is think of gene flow as the genes literally flowing in and out of the population due to migration.
something to use for genetic drift could be a small population drifting away and colonizing their own land, this would be the founder effect. I'm not sure if this helps you remember it any better but thought i'd give it a shot.
Gene flow (also known as gene migration) is the transfer of alleles or genes from one population to another.......For genetic drift,,,,,look for the word SMALL POPULATION !