Ever tried to paint your own walls before? Ever wonder how the heck the professionals get that straight line where the wall meets the ceiling? This is a trick called cutting-in (yet another of many tricks I learned form Mom). If you want to try it yourself there are a couple important things:
1) The brush. Make sure it is angled and 2"- this usually does the job best. Also, you should be spending around $10-$15 for a good brush to ensure that the bristles will not come off onto your walls. Loose and falling bristles will also make the job look a lot more sloppy.
2) Get a 6' ladder. Or if you're tall you may only need a regular paint-ladder about 3-4' tall. Either way, you want to be able to be at eye-level of where your wall meets your ceiling...this will make drawing the line much easier.
TIP: When hiring a painter (say, from craigslist), always ask if he/she "cuts-in." This is a sure sign as to whether or not they are "professional." A great painter will almost never tape-off ceilings.
The following link is pretty good...you can see how the painter sets the bristles just up against the wall where it meets the ceiling and then, at an angle, glides the brush in a straight line across the top.