I am a 155lbs blue and a 200lbs tall lanky white belt has been triangleing the shot out of me. I try to keep my elbows in. I try to pin down his leg and pass when he goes to throw it over my shoulder. I try to look at the ceiling and posture up or hide my arm from going across my body, but he is just too strong it seems like no matter what I do he can just break my grip and straighten my arm out then across. Please help! 
When he pulls your arm across, flow with the go and use the other arm to grab his knee on that side and keep turning while you drive your head to the mat, trapping that leg. Just protect the inside arm by bringing it under his leg. Then work your way out.
Don't let him control your sleeves, try standing to pass, try your pass to the other side, put your knee in the center when he opens his guard.
Just gotta keep trying. The more he catches you the more you're gonna learn and eventually you'll stop making the same mistake.
Sorry misunderstood your question, I didn't realize he has already got to triangle position. Which my first advice is avoid getting one of your arms isolated.
But the arm across is not necessary to finish the triangle, controlling your posture is. You need to sit back hard and get your knees under his butt, make him sit on your lap and keep looking to the ceiling popping his legs open with your hands on his hip like you're doing CPR.
A little defense i like:
As soon as they lock it I turn so we are perpendicular. I immediately pull the hand of the arm in in towards their crotch and posture up. At the same time my arm out is coming to the crotch and I lock my hands one elbow in one out forming a straight bar essentially across their crotch. With your hands locked they have a hard time breaking posture or pulling arm straight. This is my go to escape
This escape has worked well for me. I learned it from Ortiz's old training partner Jimmy Terrell.
http://www.youtube.com/watch?v=zE1mZsb4tmA
Starts at the 6:30 mark I believe.
What you should do is give back your blue belt until you can stop a white belt from triangling you.
robhustle - What you should do is give back your blue belt until you can stop a white belt from triangling you.
So you've never been caught by a white belt lol 
robhustle - What you should do is give back your blue belt until you can stop a white belt from triangling you.
You sound like you are super good at jiu jitsu and just generally awesome. Can you tell me how to be tuff an cool like you?
pick him up and spike him on his head.
To avoid getting in triangle in the first place... when your are trying to pin his leg, make sure that your elbow is connected to the inside of your knee. Your foot should be fairly close to his leg that you are trying to pin. Now bring your hips forward so that your shin is over his thigh. You really want good posture during this time. So his other leg (the one on your shoulder) should be more against your chest than your trapezoid. Keep your hips jammed up close to him... His hips need a path to your neck in order to triangle.
Thanks for all the help, except robhustle, you can eat shit and die.
ninjaboy562 - This escape has worked well for me. I learned it from Ortiz's old training partner Jimmy Terrell.
http://www.youtube.com/watch?v=zE1mZsb4tmA
Starts at the 6:30 mark I believe.
That's basically what I'm talking about.
This guy's counter can usually be beaten by doing the escape instantly or by bullying through.
https://www.youtube.com/watch?v=zfpjchaM1u8
I agree with Herbish1 and others about the importance of proper posture.
Think about your two knees and heels as three points on a triangle, focus on getting your head exactly in the center of that triangle. Also, tuck your tail and engage your core. Move your knees forward and shelf his butt/hips on your lap.
Suppose your right arm is trapped, if you can frame under his ribcage (his left side) with the palm of your right hand and open your left shoulder it will help create space to slip your outside (left) arm and lock your hands like BJJBuster describes.
Whatever you do, try to not let your head break the plane of his hips.
Also, please describe how he is catching you in triangles in the first place. The suggestion from mrgoodarmbar of keeping your elbow and knee together will help for sure, but otherwise, is he using different setups?
Make your hand of the trapped arm like a fish and swin that fish between your neck and his inside knee bend and get your posture back. You have to anticipate the triangle - and the timing is the the stuff...then you can worry about the various means of peeling, prying or sicsoring your way out.
Jumbo Reverse Shrimp - When he pulls your arm across, flow with the go and use the other arm to grab his knee on that side and keep turning while you drive your head to the mat, trapping that leg. Just protect the inside arm by bringing it under his leg. Then work your way out.This
"You fucked up a long time ago!"
man, we have a 200# guy at my gym and he will triangle you to living death. He is tall and strong and i am only 5'9 145#. Once he gets it locked up you dont have much of a chance. Something i have had a bit of success with is posturing up and getting a knee up in his butt and work for something like a knee up the middle pass. like right up the middle so i can pry back and the knee up the middle will kep him from locking it all the way up. But it is really a race because there is a point of no return.
First bit of advice is pass standing, dont ever want to be in his closed gaurd. His legs alone will wear you out.
Second is control his feet. Will be impossible to triangle you while controling feet.
Dont try to pass until you have his hips under control and legs pinned.
this guy at my gym has triangled me with one of my legs in! lol thats how long his legs are
this is the size difference between he and i, we are on the left with arrows.
http://i37.tinypic.com/nytrwl.jpg
