No deposit, No return.

A professional DJ always has a contract with his client.  Always. 

It protects the client in case you don't come through, and protects you in case the customer doesn't come through on their end.

In case you're wondering how to write one, you can google 'dj contract' and find plenty.  Most have the same boilerplate that basically says the 'DJ will show up at a certain time, play and be done by a certain time and will be paid the agreed upon price'.  Anything else is extra, but handy.  It can stipulate:

What the DJ will be wearing
What lights will be used
Whether smoke or fog will be used

Where the DJ will play (full address)

Whether he is allowed to drink alcohol or smoke
Whether he is allowed to take breaks
Whether he is allowed to accept requests
What the DJ is not allowed to play
Whether the DJ is considered in catering
Who provides the table for the DJ's equipment
-- and lots more.

I've seen five-page-long contracts that include all kinds of clauses...even ones about the DJ not being held responsible if the crowd doesn't dance. I manage to put all of my information on one page.  Its all up to you...but consult a lawyer if you feel that contracts are deep water for you. 

Its also very important to request a down payment or deposit when the contract is signed. 

Why?

I checked in with one of my booked bridal customers recently...and while I always try to work with a client's budget - she had to slash her entertainment completely...which meant my services would not be needed.  It's rare, but happens, and its unfortunate.  A properly worded contract can help you, though.  I (and many others) have a clause in our contract that says the deposit is kept if the customer has to back out...or refunded if the DJ backs out.  This way, the client's performance date is safely reserved with the promise of the full payment by the end of the performance. 

Its a valuable service...you wouldn't want someone to take your table away that you reserved at a restaurant simply because 'someone else wanted it.'  Plus, its not a money making scheme...its just good business sense as long as you apply the deposit to the total.  My deposit is usually 10-20% of the full contracted rate. 

Be sure and get the terms you want in writing, and make sure any customer terms are there as well.