You could also start charging interest on the overdue bill, but I believe you have to state this in your invoice. then you have an excuse to call - to update them on the amount they owe.
I've not heard of anyone at startup level resorting to a collections agency but I have heard of the small claims court being involved sometimes. Never had to go that far myself, so I can't really comment.
Most collections agencies cost you little or nothing up front, they just split whatever they collect. Hiring a lawyer or spending your valuable time deciphering legal forms is almost definitely going to cost you more up front.
Also, small claims court has caps on what you can sue for. More than a few grand and it's not an option.
If these companies are large enough to have accounts payable departments, it's often standard to keep onto company money for as long as possible. If they can eke out a few more days of interest they will.
If you're persistent (read annoying) enough, they'll want to pay just to stop you calling every day.
This worked flawlessly for me in the past but I did take in to account that I probably wouldn't be working for those customers ever again (and funny enough, in most cases I did, and in most cases I used these as reasons to demand up front payment on future jobs).
The squeaky wheel gets the grease...