dont know how it is in UK, but what I have got experience from my country.
- think about and set the direction of your event in terms of musical style you want to go. If people will feel the continuity from first party to the next one they will most likely come next time.
- be sure you have got enough sponsors (partners), who will pay some part of costs or will donate a headliner
- be sure you advertise as much as possible (flyers, posters, banners)
- be sure you have money to have proper sound/lights/deco
- dont forget to offer something extra what people usually dont see/hear/feel on other parties
as for the DJs, I think there is so many aspects affecting the price, so it is very hard to say more on accurate prices. All depends on capacity of venue, entrance fee, situation on a scene in current location, communication with DJ(agent). Some DJs are open to discussing price more than others. Eg we had very nice prices from people like Headroom, Jel Ford, Jay Denham, Dirty Bass, Oscar Mulero, who seems understand that not everything on scene is now how it used to be, but on the other hands prices from big names like Dave Clarke, Cristian Varela, Jeff Mills, Hawtin was like from different planet......