These are tricky for me, too! I don't have any good solutions for you, but you're definitely not alone in this. I'm awful with these.
I tend to not end up with the straightest seam (even if I draw lines to sew directly on) and so I end up with weird puckers and curved-up pieces when I try to press them flat. Argh! I think for ME, I'm better off cutting the triangles first and then sewing, so I can use my quarter-inch foot to keep me in line. Which is disappointing, so I keep trying to do it the more efficient way, telling myself I just need to keep practicing...but I keep getting aggravating results.
This is why most of my quilts are made with squares and rectangles! LOL