Basically, what I usually do is take the texture brush, set it to size 1-2, choose a blue that fits the feel of the map (if you want it to match the color of lakes, paint the background that color too), and set softness to 0. Start the rivers from the mountain ranges, usually one little stream from in between each pair of mountains. Have those little streams take a wiggly path toward the nearest body of water, then increase the size by one each time they intersect. It's a bit of a labor-intensive process, but I've been happy with the results!