My method is to wait - longest so far has been 5½ months, but I still got paid. I've not yet had a bad debt (but have had to stop asking for money where a job's gone a little wrong, which nearly amounts to the same thing.)
None of us likes to wait but I do find it a near fool proof method of receiving payment. In 3 months they can't have any excuses up their sleeve when you prompt them about payment.
If you won't wait then:
First stage is to make sure your "case" is waterproof. For example, does your invoice say pay within a certain period or by a certain date, does it demand full payment or is this not mentioned? Just because you do a job responsively, quickly and to the agreed price does not mean you'll get the payment quickly unless you have sorted this out in advance or show it on the invoice.
Second stage is to contact him.
If he won't talk or meet you then you have to send a letter (recorded delivery). This letter should give him reasonable time to pay, usually 7 days.
After this you need to send another letter (solicitor type letter) giving him another 7 days and warning him of possible charges and interest.
This gives him a full two weeks to pay without penalty.
I'd suggest after this the small claims court is the cheapest and most beneficial method and I think this can be done on line.
There might be a lawyers forum for further advice?
I wish you luck!
PS - if you damage his property or try taking "your" materials back, this amounts to criminal damage and you will be the loser. I know, I don't agree either, but that's the law in this country.