It might seem obvious but costume is a very important part of LARP - no matter what system you are playing - if all your characters wear whatever you come to the session in then no-one willknow who you are - and if you're a Ventrue who wanders around in Jeans and an old t-shirt then people aren't going to know you're a Ventrue. I'm not saying that you have to wear a suit - heaven forfend! My current character is a Fashion Designer, and until I became clan elder I was regulary assumed to be a Toreador - I still am to some new characters and NPC's. But it all adds to the Role-play.
If you can, try to imagine what style your character would be likely to wear. Remember though, the Masquerade can be breached even with clothoing - if you wander up Northumberland street on an evening dressed in your finest Roman Toga (cos you were embraced in the 4th C. CE.) Then people will look at you, if they look hard enough then they'll notice - aren't you a bit pale? And you don't ook like you're breathing. Try not to draw too much unwanted attention to yourself, it only causes heartache...
Being a Ventrue however is quite simple, if you're stuck then go for the Suit - it's what everyone expects anyway! Don't however wear your work suit or your best one for Weddings funerals etc... If you can then head down to the local Charity shop - you can pick up a suit for next to nowt (usually about £10-£15.) This goes for most costume, just get a cheap something - you're going to be running around and falling over so you don't want to rip that £195 Versace shirt that you got last week! If necessary just tell people what label it is if you want to make it known!
Here's another one where it's best to buy something cheap and servicable, what's the point of spending £50 on a briefcase that gets thrown accross the room in a fit of rage? Or whipping out your mobile phone to answer a call and getting it knocked out of your hand by a couple of people in the middle of a fight scene? Also be careful! Roleplay is meant to be fun, so don't bring along that Toledo steel sword that you brought back from Spain last year - you won't be allowed it! If you have a staff, or a walking stick, try to get a LARP safe one made, if you can't then make sure that you don't use it as a weapon. EVER!
The thing that I've found about costume and accessories is that they not only tell other people who you are, but they help you to get into character too!