Payment Plans for Flat Fee Work

This question is for all of you out there who work on a flat fee basis - what’s the best way to implement a payment plan for clients while still protecting yourself? Right now my flat fees are paid 50% upon hiring and 50% at the end of representation, but I’m thinking of offering flexible payment options for potential clients who may not be able to make those payments all at once. I’ve seen some firms that take a monthly payment for a few months until that first 50% is received, then they start doing the work until the second 50% is received. Does anyone have experience with this or some advice? Thanks!