Look up a basic saddle to cranks/handlebars fit guide. There are loads of them. Make sure that your knees are getting proper bend at the bottom of the pedal stroke and that your feet are on the pedal in the right spot. Your butt bones should be on the rear of the saddle. If you find yourself riding more forward or sliding off the back is good indication something isn't right.
Something else I would suggest is to look into a rack/pack and get the load off your back. It will help with saddle weight.