If I'm using milk as an ingredient in soup, how can I prevent it from curdling?
Milk curdles due to acidity, so if you can raise the pH of the soup by adding something basic, it would help, although it could affect flavor. 

The other thing to try is heating the milk to ~90 degrees C before adding it to the soup. This will cause the protein in the milk to change in a way that curdling won't result in as bad a final result--it may in fact appear that the milk hasn't curdled at all. 

You can also "temper" the milk (as mentioned by Ocaasi): add a bit of the soup to the milk first; then add the milk-soup mixture to the soup at large.  And yes, higher fat milks will tend to work better.