This can actually be one of the most lucrative and stable sources of revenue for a small software consultancy. At the "end" of a successful project, I usually p

This can actually be one of the most lucrative and stable sources of revenue for a small software consultancy. At the "end" of a successful project, I usually pitch a retainer agreement to the client. There are lots of variations on the terms, but I've generally done something like "for (~12-24) months, you buy (~8-24) of my hours guaranteed per month at a discount rate of (~10-15%) off my standard rate. Invoiced at the beginning of each month, use it or lose it (throw in a month of rollover if they push back)." It's a win-win: the average client doesn't use anywhere near their entire allotment, but in general they're happy to pay for the "insurance" that you'll be available if you need them. You're never on the hook for more than you can reasonably provide: if something bad happens and they need more than their allotment, then you can renegotiate that as a new contract. And most of all, predictable monthly revenue does wonders for your cash flow.