There are a few things you can do to ensure a comfortable fit.
1. The side Velcro should snug the unit to your face without putting pressure on it. If you feel like it's pulling the Rift against your face, it's probably too tight. When fitting a Rift, I hold it lightly up to my face and do up the side Velcro without pulling the straps. You're trying to keep the Rift secure, not pressure-weld it to your face.
2. The v at the back of the head strap should go below the protrusion of bone in the back of your skull - called the Occiptical Proturberence. When I first got my Rift I didn't relax the top strap enough to let this happen. Completely undo the top strap and make sure you pull the plastic surround strap down so it sits just below the notch in your occipital bone. If you do this correctly it will help a LOT with the feeling that the Rift is front heavy or slipping forward.
3. Tilt the headset. Not everyone realizes that you can tilt the main housing on hinges. Use tilt to balance the pressure between your forehead and cheeks. This is also a good way to ensure you have a large sweet spot in the lenses.
If you do all of this correctly the Rift should actually feel a little bit loose. That's fine. I often end a play session of several hours with only the faintest hint of 'Oculus Face'. If you find you have a clearly defined red ring on your face after playing then it's not adjusted well. This is a VR headset, not a Facehugger.
Even adjusted correctly it sounds like PSVR is a better mounting solution. I just figured it was worth helping people ensure they've done everything to make the Rift as comfortable as possible!