A new or used gallon paint can with the rim cut off makes a perfect container to load your brush from. There's no rim to get in the way when you slap the brush against the side, and no lip to get messy from dripping paint.
Brian J. Doherty, a full-time painter in Richmond, Virginia, and frequent contributor to Fine Homebuilding, demonstrates how easy it is to make a cut bucket and the advantages of using one in this video. For more painting know-how from Brian, read his article Repainting Interior Trim.
Also check out Brian's other video, How to Cut Paint in a Straight Line.