There are two approaches for inside corner trim:
The first is to mitre the corners. Since corners are rarely exactly 90 degrees, take two pieces of scrap and try different angles until you find the perfect angle. Make sure you always use the same angle on both pieces or they'll never mate right. Also, make sure the scraps are reasonably long. Walls usually aren't perfectly flat and that variation can cause very short pieces to fit together at a different angle than longer ones.
The second approach, which can take a bit more time, is to 'cope' the corners. With this approach, you'd run one piece straight into the corner, with maybe a small back-bevel to allow it to fit into a corner that's less than 90 degrees. You then 'cope' the other piece so that end of the piece has the same profile as the face of the other piece. A picture's worth a thousand words, so try this link:
Mitreing inside corners is the approach that's usually taken by less experienced folks because it's a little easier, but it rarely looks as good as coping, especially after a few seasons of expansion/contraction with humidity changes. Coping also makes you feel like a true craftsman and will impress your friends and neighbors.