LOL, true.
Here's my criteria: I look at an ad/van/website for 20 seconds, casually, not trying to memorize. An hour or two later think of it and see what i remember about it. if it's positive and not horrible...then build from there.
You probably won't be able to do this for yourself, because we have personal biases to our work, but have someone else do it and let you know, especially from your target audience.
then compare the results against other ads/vans/websites in your competing area and get rid of anything similar to what is already out there. Then keep it simple and easy to read the phone number/website addy first, name second. make sure any memorable images relate to the work you offer. An example, the sillouhette of a football would be a great logo for a sporting goods store, but not a cleaning company. While it is simple and memorable, it doesn't tie in the mind to the cleaning business.
Rinse and repeat until you get shining results.
Take care,
Lisa