Communication: You can get away with poor communication a lot in PvE. This is crippling in PvP - you don't have time to sit around typing at each other. Before you start playing Team Arena, get Ventrilo or Team speak for voice chat as appropriate depending on what your group is using Dungeon Fighter Gold.
If you're joining a pick-up group, have both installed and set up ahead of time. When using voice chat, only say things that are relevant or no one will be able to hear important calls.
Don't rely on people recognizing your voice - even using your name isn't ideal. A better method is to use the position you're in in the team - so instead of saying "I've got Spiteful [Spirit] on me!" or "Spiteful on Clamatius" say "Spiteful on 2!" (If you're second in the party window list).
The reason is that normally you're talking to someone who'll remove it and it saves them from having to look through the roster to find you. You can use the same tactic when talking about the opposition - e.g. "3 have Blackout".
That's a good habit to get into that will be rewarded when you're playing against teams that don't share your alphabet. Useful thing to say: "Migraine on 3!" Not so useful: "I totally owned that dude!"