1. Call, don't email. Then follow up with an email: Per our conversation today at XX.XX my bill from XX/XX/XXXX in the amount of XXXX will be paid on XXX as soon as you XXXX.
2. Make sure to contact everyone you worked with when you were there. If you got along, they'll be ticked their accounting department/boss/whoever hasn't paid you.
3. A lawyer is going to start by sending letters to important people in the company, but they are going to charge you for them, you can send letters to important people in the company too. Reference parts of the contract/written agreement/verbal agreement they broke (payment within XX days, payment upon receipt of invoice, etc.)