When painting with a roller, you have to use a brush to get the edges and corners where walls meet since a roller can not get right up to an edge. While this works great, you can sometimes see brush marks, or differences in the paint texture in the areas where the roller did not reach.

To make sure your roller reaches as close to the edge as possible, slide it slightly off the holder, just an inch or so. When you do this, there is less of an obstruction on the end of the roller so you can get much closer to the edge. Once you are done with the edge, you can slide the roller back into its normal position and keep on painting.