Keep brushes for watercolor and brushes for acrylics separate. Watercolor work really demands a nice soft brush that can hold lots of water; brushes used for acrylics usually end up being anything but soft and pliable. I wrap colored electrical tape around the handles of my brushes reserved for watercolor.
Try not to clean brushes by smashing them on the bottom of your water cup. (Do as I say, not as I do.) Once those those bristles get crimped, there's no going back. Detail brushes are especially fragile.
Acrylic paint likes to accumulate at the base of the ferrule (the metal piece that attaches the bristles to the handle). The paint gradually builds up until the brush is hard as a rock. When cleaning, pay particular attention to this area. Many times, a brush will seem clean but when you press on the base of the bristles, paint will come oozing out.
I use foaming hand soap to clean my acrylic brushes, working the brush into the soap in my hand and repeating until the foam stays uncolored. I have found that this soap works as good, if not better, than any commercial brush cleaner.
After cleaning, gently reshape the bristles and let the brush air dry.
Don't let brushes soak in water overnight. This loosens the glue that keeps the brush and bristles together.
Save "useless" brushes for serendipity mark-making, dry brushing, and even lettering. The ends of brush handles are great for making perfect dots and the entire handle can be loaded with paint and used to stamp "grunge-y" lines on a journal page or canvas. Ruined brushes (especially with interesting blobs of dried paint on the bristles) can also be used as hangers for artwork.