This sheet is a graphic organizer that helps students determine how to use ser and estar in either of the past tenses.
How many times have I been asked "How do you say 'I was'?" to which I reply, "There are 4 ways!" Now, I simply hand this (laminated!) sheet over to the student who asked, and they can figure out which verb and tense to use!
For the Mnemonic devices, I use DOCTOR and PLACE for ser and estar, and ICES and DROSWEAT for preterit and imperfect.