The triangle choke is actually really easy; easier than the arm bar to me. I used to have a lot of trouble getting it, but my problem was just that I didn't understand the set ups. The triangle choke actually has a lot more to do with hand fighting than hip movement (although moving your hips is important too). Any time one of his hands is behind the other, ESPECIALLY when it's coming close to getting behind your leg, he's susceptible to a triangle choke.
Try this set up; Nogueira does it, and I'm sure it works great with a gi, too. Grab both of his wrists like a kimura grip (your thumbs are pointing up and away from his hands). In gi, I'm sure you can probably make this tighter by grabbing his sleeves (but I've never trained gi, so idk). From here, he doesn't know whether you're going to try to kimura, sweep, or triangle. Your choices are to either sit up and go for a kimura/sweep, OR, simply push one of his arms further back, and on that same side, bring your leg through the space you created. Put the leg you brought through across his neck, figure-4 it in the pit of your other knee, and make sure his arm is across his throat. Triangle choke.
The most simple set up is to just push his hand to his chest and pass your leg around. For the triangle, there are endless possibilities; all you have to do is pass your leg over one of his hands.