I think there are two big issues you need to think about when organising a steampunk LARP: visuals and theme.

Visuals - "accoutrements", if you will - is vital to the steampunk genre. Without analogue machines, steam-powered tanks, ether propellers and whatnot it's just simply not steampunk. What this means for a LARP is that you'll need to supply high-quality props evocative of the style.
This is not normally a problem in D&D-style fantasy LARPs or with White Wolf games. The latter take place in "our present-day world", so the visual style is a given. The former does need some gear, but not much. For us Europeans (and by extension, the American colonials over the pond :P), quasi-medieval imagery is deeply ingrained in our cultural consciousness; therefore, we have no problem seeing the polearm behind a boffer weapon or a medieval warrior's outfit behind a cardboard cuirass and a corny green cape. Steampunk visual style, however, is not such an integral part of our visual culture, so our imagination cannot bear the same amount of burden. Therefore, you need higher quality physical props to help players jump the mental gap from "corny items" to "part of the setting". I imagine that good period costumes would be very useful; and you should probably also provide a number of visually pleasing macguffins as well.

The other issue is theme. While it possibly means slightly different things to different people, steampunk as a whole still has a highly specific thematic focus. Personally, I could best describe it as a reenactment of late 19th, early 20th century (largely) European culture, politics and history. For me, steampunk is centrally about the European Imperial Spirit - white man going to other continents (or planets) to civilise the natives and build his empires, the politics and schemes in the worldwide game for power, the indomitable spirit of inventors, explorers, traders and conquerors furthering the progress of mankind.
If you want to retain the feeling of steampunk, the game's premise and plot should conform to these genre themes. Which I guess will probably require a great deal of thinking, preparation and whatnot, but if you really want to run a stempunk game, you have your job cut out for you.