First things first, you're going to get rolled over if he's fresh simply because he's that much bigger and stronger than you. Keep doing what you're doing and as he gasses it'll be harder for him to roll you.
Another kind of mount I like to use is a bit weird. Instead of grapevining his legs or riding up high in his pits, I like to lock my feet below his hip then hip into him. At this point my knees are probably going to be off the ground but all my weight and hip is driving into his gut, making it really hard for him to move and sucking the air out of him (I've seen Renzo Gracie use this kind of position when finishing a mounted guillotine). Since your knees are not touching the ground, it's important that you spread your arms out kind of like you're Superman and teeter-totter yourself back in place like that of training wheels on a bike. Try it out and tell me if it works for you.
Hate to advertise my grappling videos again but you can kind of see me use it when I mount the guy in this video although I used my hands to lift his head so he couldn't bridge instead of having the arms out.
MySpaceTV Videos: Judo Match 1 by Mata Leão