The only honest answer I can give you is, eat healthy and exercise. If your love for coaster is that great, that motivation right there is enough for you to try and loose a couple pounds to fit on Nitro. I was once 285 with a 54 inch chest just barely making the belt on Batman The ride.
I was never worried about not fitting on a ride until I had to do the walk of shame twice at Paramount's Kings Dominion. So thanks to Shockwave and Flight of Fear I started to watch my weight. After this experience I lost 20 pounds and headed for Cedar Point for Millennium force. It took me three attempts to click the belt.This made me realize I still need to loose more weight. I still worry about not fitting on certain rides and this keeps me motivated to watch my weight.
With all that being said, The only advice or tip I can give you is look for the Big Boy Seats in either row 4 or 5 of B&M's Other than that, there are no tricks on how to fit on a certain ride. Either you do or you don't! And for some reason you did manage to fit, I can tell you from experience holding your breath and being stapled to a ride is no FUN. All you do is find yourself thinking about getting back to the station and not enjoying the ride.
Hopefully this all helps coming from a fat guy named Bubba with an avatar of White castle . The walk of Shame sucks because I have done it and I am only here to Help. Good luck with everything !!