Lap Your Cut-In Onto the Walls
If you're planning to paint the walls too, lap the paint onto the walls a little bit. Then when you paint the walls, you can err on the side of leaving a little ceiling color showing when you cut in and it won't be noticeable. Some painters like to skip this cutting-in step and save time by mashing the roller into the corner instead, but this method is sloppy, builds up excess paint in the corner and can leave runs or a thick paint line on the wall.
Don't Be Afraid of Color
You may not want to paint your ceiling yellow, but don't be afraid to deviate from plain old white. Painting the ceiling a color can make a small room seem bigger, or a room with a high ceiling seem more intimate. Plus, it's just more interesting. Ask at any full-service paint store for help in choosing complementary wall and ceiling colors, or search online for examples of rooms you like.