Setting up accounts receivable

As we grow, we're starting to get more business buying from us. Still pretty small at this point (apples for their staff, lunches, etc), but I want to start setting up our accounts receivable right. We currently assign a business account in CoPOS, then allow them to charge on an IOU account. We don't have any standard terms or contracts, but I'd like to get something in writing. Can anyone share how your AR is set up? And any advice for starting out?

Thanks!