After I shared in facebook one of my first decorated cakes in 2007, (see the cake here), I have received many inquiries about techniques to frost the corners and make them look nice and smooth. When I first started decorating cakes the corners stressed me out because they never came out perfectly smooth. I used to camouflage it with decorative borders. I kept practicing, and finally I frost the corners more relaxed and they come out smooth. Now frosting a cake is a "piece of cake" :)
What I do and what works me for me: after crumb coating the cake, place about 1/2 inch thick of meringue or any other frosting, in the corners and other dabs in the sides, then gently smooth the corners with a spatula.
So this is the trick that I works for me, I place more frosting in the corners, so that when I smooth out with a flat spatula, the corners will be equally smooth and even with the sides.
Do not let this stress you like I let it stress me in the past, after you practice several times, you'll frost around the corners without any difficulty. In all decorating techniques, meringue is always more difficult to work with, but not impossible!
I leave you with this short video, of how I frost around the corners.