Fuck they have a frigging website devoted to it know, salvia is becoming too well known for my liking.
Check erowid.org for doses and such. I would recommend the 5x extract as that give you a guide of the potency, the s-extract doesn't.
A good guide is to start with 50mg and move up with increments of 25-50mg. Be careful with Salvia, and be sure to hold as much as you can for 20-30 secs. You should be able to get roughly 100mg - 150mg in one go with a bong.