This topic has come up a few times. Did you search it

Are the old topics too old

Anyway...
I made the change 2 years ago. Have not considered changing back, ever!
Stance: When I first made the change, I simply leaned back until my weight was on the harness lines. Avoid the temptation to "sit" in your harness when the wind lulls. If you feel you need to force more weight in to your sail, push your boom away from you with both hands.
Harness lines: Experiment! At the start of last sea breeze season, I tested 20", 22" and 24" lines on my wave gear. I have stuck with 24" lines as I have found I can get more power and greater wind range from my sails. I also did same experiment for my race sails, and likewise, I found long lines to be the way to go. I now use 26" or 27" (which is not that long compared to the 30" lines others are using).
Wind strength: all

Waist harnesses probably better suited to waves then race!