i sympathise completely...me and my family alike have the memory of a piece of soggy toast. My advice is play stuff you know and like because it's sticks in your memory better. However i never have any trouble remembering stuff i write myself. So relate the tab to own material or visualise the lyrics instead. For example i have a clear visual image of the song War Pigs by Sabbath...as a result i can remember the full song tab and all.