The experience of disc jockeys varies greatly.  Find out how much experience the actual disc jockey performing at your special event will have.  Look for someone that has been a disc jockey for years, not months, and has experience performing at events like yours.  An experienced disc jockey will have performed at hundreds of events and is a master at crowd interaction and music selection.  You will pay more for an experienced disc jockey, but it's a good investment if you want a successful event.

You want to find a disc jockey that will match their interactivity level and stage personality to your event.  At an event like a company picnic or bar/bat mitzvah, you would probably look for an outgoing disc jockey that is highly interactive.  For an event like a wedding reception you will likely want a disc jockey that is less interactive and takes a more formal approach.  The disc jockey should take the time to find out exactly what type of interaction level and disc jockey personality you are looking for and match their performance to your preferences.

Check their references.

Get several references from previous clients of the disc jockey company.  Make sure the references are from recent events, preferably from events held within the last six months.  You also want to make sure the references are for the exact disc jockey that will perform at your event.  References for a disc jockey that won't personally be at your event mean absolutely nothing.  The quality of the show depends entirely on the specific disc jockey at your event.  Trust the comments of previous customers much more than a slick marketing brochure.  Don't ask if you can come to an event to "hear the dj".  Disc jockeys are employees at private events and do not have the right to invite guests.  Instead ask to meet with the disc jockey over coffee, lunch, etc. or thoroughly check their references.  

Make sure they are using professional equipment.

There is a tremendous difference in quality and reliability between home stereo equipment and professional duty sound equipment. With professional equipment you will be reasonably assured of quality sound and reliable operation.  If you like, ask the disc jockey company the make of their equipment.  If you recognize the names, and they are available at consumer electronics stores like Best Buy and Circuit City, they are using equipment designed for home use and not professional use.  Also, make sure the disc jockey has some sort of equipment backup system in place..

The disc jockey should be dressed appropriately for your special event.  You will need to ask the disc jockey exactly what they will wear to your event.  Some disc jockeys will wear a full tuxedo to a wedding reception, or possibly a nice suit. For other less formal events, they should still wear neat and tidy clothing.  Jeans and a t-shirt are not appropriate clothing for a disc jockey at any special event.

Your disc jockey should be able to entertain everyone at your event by gearing music to you and your guests. Their music library should contain everything from big band, oldies and disco to the latest dance, country and rock, and they should play a variety of music styles.  The disc jockey company should be able to provide a list of the music that they will bring to your event.

What you can expect to pay.

The disc jockey fee is affected by geographic locale, day of the week, time of the day, number of hours of music required, lighting and sound system required, and experience of the disc jockey.  On average, you'll pay $100 to $300 an hour for a disc jockey with a four-hour minimum.

Check availability early.

Top disc jockeys are typical booked a year or more in advance.  Start your search for a quality disc jockey as early as possible to ensure that you find quality entertainment.  Do not wait to select a disc jockey, as your choices will be severely limited if you wait until only a few months before your event.  You will almost always have to pay a non-refundable booking fee to the disc jockey company to reserve them for your event.

Get it in writing.

Protect yourself and get it in writing.  Make sure that you have a signed contract with the disc jockey company that spells out the date and time of your event, name of event location, hours of performance, fee schedule, deposit required, who your disc jockey will be, cancellation requirements, and when the final payment is due.