I have been doing this professionally for over 10 years now and I’ve tried them all. Try to find a 2 1/2 or 3" angle cut Purdy brush and you wont be disappointed.

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Every time I paint the thing I find most frustrating is the endless taping off of ceilings and trim, only to discover when I pull it off that in many places the paint has snuck under the tape and left a ragged edge. UGH.

This is the tip I recently learned about:
When you have all of your areas taped off, apply a layer layer of clear "paint" over it before doing the rest of your taping. I’d suggest asking the folks at your paint seller what product they’d recommend best.

http://www.thatpainterlady.com/stop-paint-from-seeping-under-painters-tape/

I won’t try to answer your original question, but this might be helpful information for painting methods etc.:
http://home.howstuffworks.com/how-to-paint-room.htm
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