The triangle can be hell if you get caught in it. Just because it’s such a fundamental part of Brazilian Jiu-Jitsu it’s rather worthwhile to invest into defending and escaping it. You’ve probably covered the basic preemptive defense of not letting your arm and neck get stuck between your partner’s or opponent’s legs. Posture is great too. However, there comes or will come a time when you will get caught. It’s inevitable. When that time happens you’ll need counter measures.
We’ve got Mr. Rener and Rorion with a live Q & A video on one of the most mechanically effective triangle escapes there is. The one thing you’ll need to pull it off is a hip frame. You’ve got to frame the hip with the trapped arm. If they put your arm across you’ll need something different, possibly the tap.
Once you’ve got the hip frame you can move with the escape even if they break posture. That’s the great thing about this escape. Most opponents will primarily be concerned with breaking posture. Give it to them and then use this to escape. Just don’t forget the hip frame. Once you’ve the frame you’ll need to swim your free arm underneath and underhook the thigh looking to pin it down into the ground. Once you’ve got this it’s almost done. Use your trapped arm to hook the knee and work from there. Watch out though. Experienced guard players will look to combine into something else, possibly a backtake or an armlock. Cheers.