Good info here already. Here's my take:
Business Licenses
It's entirely state dependent for licenses. Georgia does not require a license for DJs, bands, etc. unless you're putting on large public shows or selling tickets to your event. All states vary, but most I've done research on do not require a license for Mobile DJs. Check with your local Chamber of Commerce.
Insurance
I have an umbrella policy through my insurance company that's provisioned by Traveler's. Liability is $1 million and gear is covered at $25,000 replacement cost. I'm taking a bit of a risk there, as the speakers are clearly worth more than I listed, if I had to purchase them. The insurance is a little under $400 per year, but it has a couple stipulations: I will not be covered if I perform in bars or clubs with my gear unless it's a private event, all events must be contracted, and I cannot leave the gear in the trailer or vehicle overnight.
Your trailer should be covered by the business policy. You don't want your car insurance to find out you used the trailer for business... they likely squirm their way out of paying for it.
Business Entity
I also run as a sole proprietor for ease of taxes. I have done my own taxes for a long time, as the one time I did use a CPA, it didn't pan out to be a very good ROI. With my 9-5er and my wife's income, there's very little incentive for me to establish a LLC - I just don't make enough with the DJ business to make up for the time and money spent on corporate taxes and maintenance - other than the risk mitigation. The way I see it, yes, a LLC can protect your personal assets, but litigation costs can still drain you for everything.
If you itemize, you can still deduct any and all business expenses as a Sole Proprietor. One thing I see becoming a problem is business checking. Stupid banks are starting to charge for former "free checking" accounts. I have to do some more research, as my SunTrust account is going to turn into a $12/mo charge.
Monthly Expenses
My only significant monthly cost is music. I pay for a couple music services: ERG, Promo Only, Prime Cuts, and a variety of remix services offer monthly delivery of discs. There's also a bunch of good record pools out there if you actually mix during your sets. Although these are regarded as marginally legal, I often am using the same exact songs that I have on one of the monthly services, just different remixes/intros/instrumentals of them. Something I used to do myself, I now basically pay someone else to do. We're talking around $60/mo total for music.
Occasionally I'll have a gig (karaoke, DJ, etc) at a bar that I'll promote for. I'm a new media guy by trade, so I do some artwork myself and most of the time I use GotPrint for the glossy prints.
GotPrint is also excellent for business cards - cheap and outstanding quality. I go through about 2000 cards per year. At charity gigs, I'll often print up some stickers for the backs offering 10% off for xxx charity members and leave a bunch of them on tables and with the organizers. I also distribute at the venues I frequently perform, the city halls I work with, and various formalwear and wedding providers.
Only other I can think of is my website, but I pay that yearly. The two providers I use regularly are Omnis and Fatcow. I don't update it nearly as much as I need to, though... slacker.
On a side note, I don't know why folks have an idea that I'm a hugely successful DJ... my revenues have only averaged $15k per year! There are plenty of guys out there doing way more business than me. I just have fun doing it and I try to give people a professional, affordable entertainment. It's my creative outlet from the daily grind, since I'm not good enough to be in a band
