There are many talented, award-winning painters out there whose bases are superb, but I have come to a series of techniques that gets good-looking results in a very short time.
After you have glued your models to the base (in these examples we are using American M5 3" guns) score the base using an exacto-knife. Just a few scratches will do.
Then I mix up some tile mortar (I buy it dry and then mix it with water) and use a beat-up old drybrush to spread the mixture around the base. The primary goal is to reduce the edging that the models have around their feet. The putty doesn't have to get on top of that little base, just as long as it reaches up the sides.
Be sure to checkout Parker's great FoW Hobby blog over at D Minus One -MBG