I'm a bit of a nut about cleaning brushes, but I also have brushes that are 15 years old and look and work like new. In the middle of painting right now with one, its like an old friend. I have several rules (which drive my brother crazy, he is of the camp that you buy cheap brushes and throw them away). As mentioned, buy the best brush you can find, stainless steel ferrules are preferred, Purdy is good, BM has a premium brush line that is good too. Second, keep the packaging sleeve and store the brushes in them after use. Third, when you start, soak the brush in the solvent of the finish you are applying, it helps with paint not getting stuck up in the bristles near the ferrule. Fourth, when the brush is starting to look like it is getting crusty, pause in your job and clean the brush - you get a better finish and stop buildup in the brush that you have to clean out later. Finally, when done, thoroughly clean the brush with the solvent (latex, use warm water and soap), taking the time to get all the paint off of the bristles, I use a plastic pot scrubber, flatten the bristles in the sink and scrape until gone.
When interior painting with rollers, which I am in the middle of right now, I also clean the rollers, you can get at least 4 uses out of them before the nap wears out, but that involves soaking, washing with soap, etc. If I'm putting on two coats in one day, wrapping the roller in a plastic grocery bag between coats keeps the roller from drying out and its ready for immediate use. This tool, for removing water, helps and I use it in a 5 gallon bucket to catch the water:
you put the roller on the end and pump the handle and it spins; this can also be used with brushes.
Sorry for the long post, its just that I got a "thing" about keeping brushes clean.