Is there a trick to getting the corners to match?

pin, pin, patience, pin, pin, patience, pin, pin, cross your fingers, pin, pin

Just take your time and don't rush it. It take practice and patience. Not all of my corners match all the time and when they are WAAAAY off, a little applique over the corners hides it very well. :)

Ditto!

You forgot cuss, cuss, pray, cuss, cuss, beg.....

Dont know if the trick I use is "legal" but a alot times I will put in 3-4 stitches at each joint I want to match and then go back and sew the entire seam. That way I can nudge and fudge along the way but still have my corners matching. That many any sense?

I always make sure my seams are pressed in opposite directions so they lock together. This has made all the difference for me. And I also pin. I would rather take the time to pin than tear it out later.

Double post. Sorry.

Yes, press the seams in opposite directions. Then they kind of nestle together when you match them up. Once you've got them where you want them, carefully pin it securely. Once I've got them pinned, I sometimes pull the hanging pieces of fabric apart to take a peek and make sure the seams line up all the way down, not just at the top where they'll be sewn- looking for a nice neat line!

Big yes to all of the above...heavy on the patience and cussing!

Press your seams to one side. With experience, you will learn which way to press your seams so that they butt up to each other and don't overlap. I actually use two pins at the join... one before the seam and one after. While sewing, these two pins help keep the fabric movement to a minimum and the seams line up perfectly. But I can't stress the pressing enough.