As someone who exclusively plays characters that are evil and live on the Surface...
- communicate with the 'other side'. If you go up to do nefarious stuff on the surface, ensure that it's as part of a storyline rather than just some weird power fantasy. Don't participate in multiple raids in a short period of time, it becomes mind-numbingly dull for us and quickly loses any meaning. That might sound entitled as a surfacer, but remember that we feed into your RP more than you feed into ours - we're the ones meant to be emoting fear before your characters, we're the ones meant to be dropping all of our current storylines to come sprinting to be a part of yours if you attack the surface. It's great fun, but only in moderation. If it happens too often it makes us feel like NPCs.
Overall, the relationship between Surface good and UD evil needs to be about give-and-take, or it doesn't work. Neither side should be winning all the time, and both sides should be willing to acknowledge the other's victories. Still, I'm not really sure there should be 'sides' at all and I'm of the opinion that War RP doesn't work at all on Arelith, because nobody can die so nobody can lose. It would be far, far cooler to see more small-scale antagonistic storylines. I have no idea how that would work, though, and hey, that might already be going on and I just don't know about it
- find your character's motivations. Nobody except the completely insane wake up every morning thinking they're the bad guy. When you're RPing, you should be so deep in your character's motivations that you think they're right
- remember that you are the antagonist. Arelith is a story, and in stories evil eventually loses.
Something that I see often is evil characters being played as evil in every aspect, all the way down to politeness. This is probably the biggest advice I have. There are many different kinds of evil. Total psychopathic moral depravity should be incredibly rare, probably more rare even than total goodness. Evil characters can be friendly, kind, loving, warm, regretful, polite, fearful, anxious. They don't all need to be power-crazed monsters who take every opportunity to attack and insult everyone around them. They don't even need to be unpleasant to talk to.