Well, here's two ideas:
1. I use a piece of sheet metal and do all my cutting in with a 3 inch paint roller. I just wrap some painter's masking tape around the business end of a piece of sheet metal about 18 inches long and a foot wide, bend the taped end into the corner and paint up to it with the roller, similar to the way Blondesense described. Never start with a fully loaded roller right next to the sheet metal or you'll squeeze paint under it onto the side you're trying to avoid getting paint on. Also, eventually your sheet metal will get all gunked up with partially dried paint. When that happens just pull the masking tape off and put new masking tape on, and you're good to go.
Now, when I paint I'm almost always repainting an apartment the same colour, so a sharp edge isn't important to me because the ceilings and walls are the same colour, only a different gloss. But, if the roller/sheet metal method results in a fuzzy line between two colours, you could always then correct the problem by using a brush... and artist's paint brush, which is much smaller and therefore allows you much more control over the brush. However, if you go this route, I think it would be better to do the cutting in with the artist's paint brush first, allow time to dry, and then paint up to that corner with the roller and sheet metal.
2. You need to be using your sash brush properly. Lots of people think that the reason why a sash brush is cut at an angle is so that the longer bristles will reach into the corner, and that's completely wrong. In fact, the correct way of understanding it is that a sash brush has the bristles on one side of the brush cut SHORTER so they won't flare out to the side when the brush is being used properly as depicted in this photo:
You see. When cutting-in a horizontal line (like the corner between a wall and a ceilng), the sash brush is held horizontally with the shorter bristles at the leading edge of the brush. If you were to use a regular paint brush in the same fashion the bristles at the leading edge of the brush would flare out more, resulting in your making a mess of the line. By having the bristles on one side cut shorter, and if you hold the brush at the proper angle, you have the same amount of flaring out of the bristles all along the width of the brush, and that allows you to paint a much sharper line with either the top or bottom of the brush.
The above web page also recommends pushing the brush away from you as you exhale. I think it would be easy to get into the habit of doing the cutting in when you exhale and pulling the brush back as you inhale.
Rots O' Ruck