Start by checking that you're using an opaque color, not a transparent one. The tube should tell you, or you can test for yourself. Flat color is easier to achieve with opaque colors than transparent ones.
You can also mix a little of a strongly opaque color, such as titanium white or titanium buff, with the transparent color to produce a color that will spread more evenly. If the resulting color isn't intense enough, wait until it's dry and then glaze over with the transparent color.
Another technique to try is to blend the paint by going over it with a very large, soft brush, before it's completely dry. If the paint is drying faster than you can blend it, try using a larger brush or wetting the canvas first (either with a brush or with a spray bottle) before you paint.
If getting flat color with acrylics completely defeats you, try switching to gouache (though this, of course, doesn't have the waterproof qualities of acrylic).
Thanks to members of the Painting Forum for various tips used in this FAQ.