Before using any of my brushes with acrylic paints, I always wet them first, then blot off any excess water before touching them to the paint. Never dip a dry brush into acrylic paint. You may never get the paint off it.
I usually pre-mix my colors and rarely use my small sable brushes to do any direct mixing of colors. That will help prevent them from becoming so loaded with paint that the paint works up into the ferrule.
I blot off excess paint before I touch the brush to the painting surface. That will also help prevent excess build up of paint in the brush.
I rinse the paint off in clear water and blot off any paint onto a paper towel before setting the brush down to pick up another brush.
I keep my brushes moist, but not soaked, when I'm working with other brushes by resting them in a special little tray that I made up myself just for this purpose. If you keep the brushes soaking in water (i.e., submerged) you run the risk of having the brush become so waterlogged that the water begins to soften the glue up inside the ferrule. In really bad cases, the wooden handles may begin to crack, as well. I use a small shallow plastic tray that is about 1 1/2 inches in depth, 8 inches long, and about 4 1/2 inches wide. I use a small rectangular household sponge that has been cut to fit in one end of this tray. The upper surface of the sponge is about a quarter of an inch below the sides of the tray. I soak the sponge in water and squeeze a bit of it out before placing it into the tray. When my brushes are not in use during a painting session, I rest them in this tray with the handles resting on one end of the tray and the hair/bristle ends resting on the sponge. The angle is just right so that the brush heads make contact with the moist sponge and keep them from drying out, but they do not get bent out of shape. Since adopting this system many years ago, I have not lost a single brush to water damage or having the paint dry on it.
When I'm done painting for the day, I give the brushes a thorough but gentle washing and use my thumb nail to work in between the brush hairs to help work out any paint residue. After a good rinse, I reshape them and lay them flat to dry.
One of the best ways to avoid getting paint up into the ferrule of the brush is to avoid overloading it or using the brush to mix up your colors. I try to never overload my brushes and use either a small painting knife or even some wooden Popsicle sticks to mix quantities of paint.
I hope that helps.
Here is the brush tray with sponge and a few sample brushes to show that it works equally well with long handled brushes or short handled ones.
Note: the sponge and brushes in this photo are completely dry. I didn't want to wet them just for this photo. The sponge contracts a bit when it's dry like this, so it would plump up a bit more when wet and in use. I cut it wide enough to touch the sides of the tray and prevent any brushes from slipping off the ends of the sponge. The slight bit of rim around the tray that is higher than the sponge keeps the brushes from rolling off the tray. You can make up a rig like this in a size to suit your needs. You just want the angle of the brush to rest its head on the sponge without bending.