Push the waist belt strap of saddle underneath your arse. Most of the saddle designs, trying to keep everyone happy, make saddles far to wide and you start out with one weight bearing strap on the small of your back and one under your rear end. This will promote moving up as gravity is pushing you down. Push them both under your rear end once you are set up.
There is not such thing as a saddle that you can wear for hours without making slight adjustments. I think so many new folks are coming into the saddle game with unrealistic expectations. If you want comfort and like to sit and don't want to adjust from time to time - get a Guido's web. If you want a lightweight saddle, that is packable, weighs next to nothing, can get in most any tree, then get a mantis - but just realize you will make adjustments throughout a long sit and you learn to deal with the "pressure points.'
Also, the more you weigh the more pressure you will feel. I had to make an honest assessment 2 years ago - I was FAT. I changed my lifestyle, dropped 40 pounds, added in some weight and core training, and that is helping a lot. I see so many folks worrying about ounces when the biggest weight reduction/savings for many of use lies between our waist and chest.