Do: Be sure to cling to the base traits of the race you're representing. An elf is still very different to a human even if they've lived alongside each other for a few years, after all.
This. So, so much this. I've seen people try to play their Elves running along a similar concept to Sav, but they end up playing their Elf as an Orc. By all means, play an Elf that's been alienated from their own society and as a result become attracted to the 'freedom' that Orgrimmar and loyalty to the Horde offers, but don't forget that, no matter how alienated your Elf has become, they're still going to have some ingrained Elven traits. Sav, for instance, has an instinctively fierce sense of self survival, which is something I most definately see as ingrained into the Blood Elven mindset. This instinct comes above honour and loyalty for him. He's cruel and ruthless, but not in the sense that an Orc would be. An Orc respects a worthy enemy, and honours him after they're defeated. Sav butchers the enemy and spits on their corpse. Just something to keep in mind for those who, like myself, are playing an 'Orgrimmar-Elf' concept. Your Elf is never going to quite blend in with Orcish society, not really.
And as for some more Dos and Don'ts....Do:
Keep in mind the logistics of a battle during RP-PvP. You cannot, realistically, just walk straight through someone. If they've got a solid line set up, there's no way in hell you're going to break through it unless you physically shatter their defenses by taking down some of the soldiers in the line. You can't just fight through them and get straight to their menders. This especially applies if you're a rogue. You can't just go invisi-mode and sneak straight through them to magically reappear on the other side. Don't:
Be afraid to ask for advice on a concept if you need it. If you're unsure of how something will be recieved, ask your guildmates, or even someone on the forums. But be prepared for some rather frank criticism. If you know you're going to be hurt by someone saying they dislike your concept, then it's probably best that you -don't- ask for help/advice on it.