For my lawns i send out a monthly invoice. Trying to collect from people can be time consuming in itself.
If im doing any side work all give him a invoice and expect to be paid on the spot with a check or cash or i give him a couple weeks to mail it. Any really big jobs i have i like to get paid in portions that way i dont feel like i might get screwed. If your gonna have help and start pocketing cash your gonna start wonder where all your moneys going.