You've probably got to be plotting to open/worship/bleed on/rescue someone from/etc the hellmouth. Probably needs more than your average vamp strength too. Or some kind of kick arse power so when you first encounter the slayer you can send her running off, nursing her wounds. (But don't kill her just yet, of course, despite the fact that at that moment you have the upperhand. No, wait for her to rally the troops and come back for you.) Harrass them for a while, but don't kill them. Kill a few minor characters. Keep your distance while you plot and scheme. Don't keep it a total secret though. And then when you're just about to end the world or whatever, the Scoobys will show up and kill you.
Basically I think if you give the Slayer something to worry about, you get to be the Big Bad. Also, witty one liners and laughing in the face of danger seems to me to make a cooler big bad.