If it's metal, pin it.
If it's plastic, use a small file and clear away the inside of the join and the ball of the torso. Make sure they have a clean plastic-to-plastic connection.
Apply a small amount of plastic cement (a tiny drop or three will do the trick). You'll need to support the model for a short while. The plastic cement will melt the two plastic surfaces lightly (which is why you don't use a lot) and they'll chemically bond.
Once bonded, as mentioned above, the join will be stronger than the actual plastic itself (so make sure you pose it the way you want when it's setting!)
You're looking for Testors or Model Masters since you're in the United States (any hobby shop with a model section sells this)