I've had some requests to do a tutorial on inside and outside corners. I don't have anything currently in process, so here are some pictures from the archive. (When will I ever learn to get a manicure BEFORE taking pictures of my hands? Sheesh!)
Make a snip as long as your seam allowance (1/8" if you usually tuck under 1/8", 5mm for 5mm, etc), and and evenly splitting the angle your are turning. In this example it's a 90 degree angle so it'll be a 45 degree snip. You may need to make this snip a bit longer, but it's easier to do that than regret you went too deep. Normally, I wait to snip until I've stitched up to an inch from the inside point.
Let me know if things aren't clear and I'll edit accordingly.