I hired a gentleman to do some construction work, paid him $500 cash in 3 payments, he did most of the work and I can not get in touch with him (his phone # is deactivated). I only have his first name and he did not give me a receipt. How can I write this off on my taxes? I do have receipts for construction materials I paid for that he installed.

Truthfully,write it off just like you have the receipt,if the IRS questions it all they can do is take it off as a deduction.

In the future, what would I need to get from an independent contractor to prove I paid them? Just a simple receipt? Do I need their full name and SSN on it?

Not necessarily,a paycheck(personal or business) is all you'd need,with their signature cashing it you have proof of your end of the agreement(labor,etc)

What documentation do I need if I'm paying them cash?

Either you don't write it off,or contact an accountant to find cash issues and independent workers.

Have the contractor issue you a dated receipt with his name and address on it.