When you go to a job site every week and spend a few hours there, it is easy to see things that can be done for the customer. This is particularly true of a residential customer. You may see a tree branch that should be taken down or removed from the property or a section of a fence that could use a few nails. Part of your maintenance on the account is to visit with the customer either to deliver the invoice, discuss future work or pick up a check.
If you offer to take care of that little job that you or your crew noticed for free, that can endear you to that customer. That little free service will come back to you in customer loyalty and in many referrals that will serve to grow your business simply because you took the time to be nice and do something for free for the good customers that you have.