Ride as hard as you can. Make sure you are pushing yourself hard enough to get benefit and to improve, but also hard enough that you don't die in the first 15min.
Stay smooth. Faster bunches tend to be smoother. I find you can ride for longer if things are nice and smooth, even if the pace is seriously fast. People not paying attention, lacking experience, lacking the ability all cause the rubber banding of the bunch and drain your precious energy trying to constantly accelerate your bulk. Communicate, assist and advise to make the ride more enjoyable for everyone.
Pick your position. As a 6'5" behemoth I always pick out a decently larger rider in the bunch to position myself behind. Anyone that sits on me gets a free ride, so get yourself as much help as is possible. Sit on the back momentarily and find your target if you don't manage to get them from the start.
Downhills - get to the front! You'll be a lovely draft for anyone else in the bunch, and you will get to carry all that precious momentum into whatever follows. Sitting in on the downs and getting stuck behind someone smaller costs big energy when you have to be braking to hold position rather than carrying your valuable momentum, especially if there is an up after that down.
Heading up a hill, try and get yourself to the front. As mentioned by many, aside from serious uphills, that should see you still in touch with the bunch at the top. Additional to this, work on being smooth up the hill. Pick a cadence and gear and try and stick with it (if it's a steady climb). The first time I rode with a rider using a PM up a hill that normally would kill me, he reeled me back from pushing into the start of the hill. He set the pace at a steady power and made me stay with him. I set a PR on that climb that day and also had more energy to hit the gas over the top. If you have a PM, try and hold steady power for the climb.
Things must be different where you ride
pdlamb 
but this move would be like cutting your own throat where I live. Poke that bear and certain death is soon coming your way! A decent lightweight climber will absolutely smoke you on a climb, no matter how much power you have. If you somehow manage to hang on, you will definitely be done for after the top.
Know your limits! There's nothing worse than being in a fast paceline and there's that rider that doesn't know when to tap out. Try really REALLY hard not to be that rider, but you seriously may have to be that rider at some point to learn your limits. In the rotation, drop out and sit on the back. You might be able to recover there and get back in for another go, but in my experience with the riders in our bunches, that is the last hurrah before dropping off. Hang on for as long as humanly possible. Dying but working in that bunch draft is 100000 times better than dropping off and going it alone. Also if you're close to tapping out and someone else does, drop with them. It's a far more enjoyable ride back either working with someone else, or having someone to chat to for the remainder of the ride. Far better than you both riding back in a solo world of hurt!