Tip - half square triangles
I have a huge number of HSTs to do. I usually make them a bit larger and use my June Tailor rulers to line up the diagonal and trim them down to size (I press the seams to one side). I now put a double layer of tape (it's a medical tape that keeps rulers from slipping, but I don't remember what it's called--but any tape could do here), and it enables me to slide the HST right into the tape so that it lines up with the diagonal seam allowance really easily. Then I use the ruler to trim--I find that the tape makes the whole process go faster and more accurately.