How can I clean a paintbrush used in acrylic paint? When I try to look up videos or tips for painting with acrylic, I always see them dipping the paintbrush into water to clean.  But every time I paint with acrylic paint, it seems there is a never ending supply of paint in my brush. Am I worrying too much about over-cleaning it? I don't try to add too much paint and I dip my brush in water to make it a little thinner so it isn't so gloopy. At least that is what I have been told to do when working with acrylic. I've been told paint thinner isn't necessary. Is there a technique I can use to clean my paintbrushes fast and effectively?

One thing I should add, I don't mean after I'm done using them. What I really am aiming at is needing to use the same size brush but with a different color. For example, my whole background is red. Well, then I have to clean out all the red because I need to use that same paintbrush for another color like yellow. I want my red brush to clean quickly so that I can get into the yellow and paint a different area. But cleaning the red out seems to take FOREVER! I dip in water, try to clean, squeeze out into a paper towel, and then use some construction paper to see how clean it is.  Basically, it looks like I'm painting with just a slightly thinner coat of paint. I'd like to clean my brush without having to clean it several different times. This leads also into the fact that I would like to paint more than clean my brushes.
Dipping your brush into water isn't going to do anything other than get your brush wet. To actually clean a brush, you need physical agitation, and I don't mean "swish it around in the water jar a few times" agitation, but "mash it back and forth against your other hand while holding it under running water" agitation. Use some paint brush soap (or plain old hand soap) and really work up a lather. Rinse and repeat until the lather bears no trace of the pigment you were using, then rinse thoroughly.

If this sounds like too much work when all you want to do is switch pigments, there are two things that will help. One, don't put so much paint on your brush: put the paint on just the tip, not the whole thing. Of course, that's not a terribly effective way to paint a large area such as a background, which brings me to: Two, get more brushes. Basically, use one brush per color, or at least per color family. If you're switching between, say, vermilion and cadmium red, you can get away with just swishing the brush around in the water jar and wiping on a paper towel. You still need to do the thorough washing as soon as possible after you're done, though, since acrylic paint can dry on a brush even if it's sitting in water (plus, sitting in water is itself a bad thing for a brush).