We invoice after the service is completed and payment is due at that time and have never had anyone request otherwise. If no payment by next service, there is no next service.
Exmarkking makes a good point. The way I see it is if you let them pay slow all the time this will end up being the standard and they will continue to pay at their leisure.
Right now I am not enforcing any late fees but I am considering it after having one acct. go 24 days past due. Normally I wouldn't let an account go that far but the customer was with us last season with no payment issues. I just figure sometimes things happen and payments might be slow occassionally.