When I first introduce the concept of the Preterite vs. Imperfect in class, I set up a time with a colleague to come into my class while my back is turned and "steal" something. When I turn around and my students tell me what happened, I feign shock and tell them that we will need to write up an incident report and turn it in to the office. That's when I hand each of them one of these forms. :) You could adapt the "incident" to be anything you want. This form is general.
Students begin to see how describing the scene is usually in the Imperfect and describing the specific actions is usually in the Preterite.
I use DUWIT (Imperfect) and DISC (Preterite) but this is in Word Doc format so you can replace this with your own acronym. You can also add your school logo, if you would like, in order to make this more "official" looking.
Also included is a poster that I put up that illustrates DISC vs. DUWIT.