There have been some good suggestions. My first quilt was all triangles (different stars). I learned a lot doing it. Some sort of sizing, starch, etc. Press, don't iron. You also might need to check your seams to make sure they are consistent. If you are not seaming straight things may not lay very flat. I know people who use a foundation like Thangles or paper piecing to get great triangular pieces. You might want to play around with a few of the ideas. I would start with seams and pressing since those are free.
I would also invest in a walking foot if you don't have one. They can really make a difference. Mine is built into my machine (if you have a Pfaff you probably already have one). Hope this helps.