Gregg has the right idea, spreading the sales.
Waiting for the magazine money was a pain, but once you got into the flow you were earnin on an investment you made 6 months back, That is fast in normal businesses.
But what a return on your money.
Then selling the set again and again provides a good buffer. But you had to be good, shooting film is a pain, selling to a client who will pay you the same money whether you are a pro or a newbie means your work has to be as good as the pros and the buyer is looking for the best 10 sets he can find monthly. It is a tough game and very few players in it. But the rewards were and are good.
But retaining rights will pay you more, ASSUMING YOUR STUFF IS GOOD ENOUGH. And this is the big problem, few of the people I get send me content can shoot.