I started with a Dakine Fusion seat and thought it was ok, but wanted a bit more freedom for toeside etc, so I bought a Flying Objects Flight Control waist harness last season.
I tried it a couple of times but didn't really like it, it kept riding up and just didn't feel right, the hook sits up higher which made it harder to reach the depower and I just didn't seem to be as balanced for jumping etc, especially when overpowered.
Over the next couple of months I mainly used my Fusion seat but kept persisting with trying the waist aswell, mainly when I knew I was having a short session as I seemed to get tired quicker and could feel it in my lower back.
Well by the start of this season I was only using my waist harness, I went on a kiting holiday in October and had about 4+ hours on the water every day for a week in 20 - 30knots with absolutely no back pain and no harness rash whatsoever.
I have not used my seat harness since and am not likely to ever use it again, the waisty is just much more comfortable and gives you so much more freedom.
I find that it hardly ever rides up even though I wear it looser now than when I started wearing it (so it will slide around when I ride toeside) and if it does ride up I just fly the kite real low, lean back and just wriggle a bit and it actually comes down or sometimes I just hook my thumb in the handle at the back and give it a little pull down.
I think what helped was that as your kite skills improve you can get away with a much smaller kite and you tend to ride with it a lot lower.
So my advice would be to persevere with the waist, you will eventually love it and if you don't I'm guessing you don't have a Flying Objects.