I would say try selling your direct debits once they are on your books and make it very clear that you are charging double or whatever for a first clean if that's what you do and wait for them to agree to paying double before you go ahead and clean them. Trust your gut instinct, i.e. locality, age of person, appearance, mannerism, etc , as you get a good feel for who is going to mess you around before you even start and those hesistant ones who don't give like to pay using bank accounts and only pay by cash are the most troublesome as a rule.
Some people despise not being in control of their finances, (me included) and are very reluctant to sign direct debit forms. They still pay all their bills mostly on time but they remain in control of their money going out of their bank accounts. This is the reason why I let people pay me by Bank transfer or Bill payments as they are still in control of their finances and it doesn't feel like you are hard selling to them. You do get the odd, dippy person forget to pay you within a set time but you drop them a reminder or collect double next time and ask them politely to pay more promptly.