I deal with two huge multinationals - they pay regular as clockwork 30 days straight into my account - and so do all my commercials. I find out the name of the accounts payable person, visit or phone and say " it's only (eg) £100, can you make sure it goes out on time cuz my kids need shoes." Usually this gets a light laugh and I get paid.
One domestic guy - after 3 months worth I put a note through saying my kids were starving, needed new shoes and even worse I was short of beer tokens, please pay up! (with a smiley) I was paid by return of post!
Be cheeky and chirpy to start with - and only then get heavy afterwards (if you need to).