If you're using wedge top bars be sure and remove the wedge before stapling otherwise you may end up stapling the wedge to the top bar. For the staple that goes through the side bar and into the top bar place the staple about 1/4" or so from the end of the side bar so that there is wood left. If you staple too close to the top bar then there won't be much wood in the side bar for the staple to hold to. Place the staple a bit off-center so that the staple will go into the top bar where the thicker wood is (where the wedge isn't removed). Naturally you'll need to aim the staple at a bit of an angle going into the wood.
At least that's how I do it, but I'm no pro.