The best way to clean your coroplast and vinyl banners is to use a sponge and dishwashing soap like Dawn. Do not scrub too hard and definitely do NOT use a pressure washer. I learned that lesson the hard way; a pressure washer will definitely clean your sign as well as taking off all of the ink :(.
If you do not leave your signs up year-round, be sure to wipe them down to remove all moisture before storing them. This is particularly true if you roll up your vinyl banners and/or store your signs where they have limited air flow.