We have started to charge a monthly fee where the customer can pay for our "extra" IT/SE services or they can buy blocks of time.
At the end of a deal, when we are signing paperwork, we inform the customer that we will install everything (including softwares) at no charge, but that we suggest they purchase a monthly contract in case something happens months or years down the line. This is in addition to our normal maintenance contract, which we make sure the customer knows is for the hardware only.
Everybody has been very receptive so far and we have been able to bring in some extra revenue for service for services we have already been providing. We don't even talk about this service until the end of a deal - kind of like when you buy something from Sears and they drop the extended warranty on you... If anybody would like any details please let me know.