Get it written down. Plan what you need to do, work out what it will cost to do it all. Work out what the worst case cost would be - what if nobody turned up at all ? Can you afford to lose that much? Who all do you need on the night - someone taking tickets on the door, soundman, stage crew, DJ - how do they get paid ?

Get contracts in place for venue, PA, lights, bands, everyone. Consider paying the bands a minimum fee with a share of profits - how will you manage that? Guarantee £100, and if 200 people pay at the door, an extra £50 for each band ?

What's your fallback plan - when the venue cancels, the PA cancels, the soundman breaks his leg the day before the show, the headline band are offered a better paying gig and play Edinburgh instead ? When the DJ just doesn't turn up and doesn't answer his mobile ?

How will you publicise it ? Posters ? Website ? Flyers at similar gigs ?

How much time do you need to devote to it and who do you need to help you ?

Could someone else do it for you ? How much of the mechanics do you want to do yourself, and how much could you hire someone to do.

Send me an email if you want to discuss further - you've got my address.

And remember, as Hunter S Thompson told us, "The music business is a cruel and shallow money trench, a long plastic hallway where thieves and pimps run free, and good men die like dogs. There's also a negative side."

Good luck!

but well done for doing it. If it's in the locale of the Thames Valley Massive (TVM) we could be the first mingle with official bands and everything.

But I will come. I'd even help out a bit. I could do the t-shirt stall..

...build it up as a gig music lovers will want to go to - then everyone the bands bring is a bonus.
Make use of every free listing there is ie I buy the Saturday Guardian for the gig guide - free as is the NME. Target places of interest for the posters ie CD shops, cafes anywhere that people who like the type of music on offer will gather.

Read the paperwork really, really carefully. A few years ago, a friend of a friend put on a series of gigs featuring a well-known but somewhat cultish popster. As it turned out, the tickets didn't sell quite as well as they expected and they were left with a fairly hefty shortfall which he ended up wearing.

He was a moderately experienced theatre promoter, but had never done any music stuff before and, among other things, hadn't anticipated the level of “hospitality” that could be got through by a bunch of old rockers playing to a home crowd over the course of a week or so.

It was entirely his own fault for being so incredibly slapdash with the contracts and assuming that everyone would be as nice and reasonable as he was, but as I heard it that little adventure put a fifteen grand hole in his bank balance.

With my band in September. We're tired of being ripped off by promoters (they like to vanish halfway through the headline set) and/or being put on a five-band bill including a rude metal band, a rubbish synth-pop duo and an acoustic guitarist who wouldn't say boo to a goose (or a 'please come along' to a friend).
Our set up:
Keep the acts of a good quality, who can all be relied upon to bring a bunch of people. It's not a 'breadhead' thing if you know them - you don't have to mention numbers, just impress upon them the need to do their bit to make it a success.
Advertising is important - also flyer a selected mailing list (email details and a pdf a few weeks before, then follow it up in the week of the show)*.
We're also having at a compére and a raffle. As the Word in Your Ear gigs showed, a little bit of window dressing costs very little and makes a big impression. It lifts it from 'some bands playing in a backroom' into an event.
Get a good soundperson, and make sure the bands are nice to them.
If you're not taking the money yourself, make sure the person on the door is both charming and utterly inflexible, especially if a lot of your mates are coming.
Never book a venue on a night when England are playing football - even the meaningless friendlies.
And as said before, budget for a worst-case scenario.

* If you need a poster/flyer designed, I'll do it for nowt. Here's some of my stuff:
http://www.gigposters.com/designerposters/98454/1_Jon_Heal.html

...the venue is free to him and I think it pays him a bit of bunce. What he has now started is give the bands tickets with a face value it is then up to them to sell at that value or discount some or all or give them away. The choice is theirs but all the cash they take they keep.

so far everyone, I really appreciate it!

...most charitable, one or two 'commercial'. All of them labour of love, like your own planned event. All bar two or three hacve been profitable, a couple more 'break-even' and probably most yielding a modest profit. Truth is, I don't really have the ruthless edge needed to be good at it commercially - but for what it's worth (along with all the excellent advice already given), here's a couple of nuggets:

1. Go 'small', if possible - its much easier to sell a 50-100 capacity space than it is a 100-300 space, for instance, plus a packed rather than half-full venue really helps the atmosphere.

2. If you hire a venue without an alcohol license, you can get around this by selling raffle tickets with a free bottled beer/glass of wine attached! It's a great loophole - used by a regular 60-capacity folk/jazz/blues/troubadour venue in Belfast, which operates as a shop during the day - and it can mean the difference between profit and loss on the night.