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.
cooking.stackexchange.com/users/220/kevins
Try adding the milk separately to only a single cup of broth.
Stir thoroughly.
Then slowly add this to the pot.
It will allow the milk to come up to temperature gradually in a less acidic environment.
Also, higher fat milk will be less likely to curdle, in case when you said milk you meant something other than milk .
cooking.stackexchange.com/users/1443/Ocaasi
You might also try using cream instead of milk; I'm not sure I remember the technical reasons
but it is much less likely to curdle.
You would want to use less of it than the milk, but it won't introduce the possibly unwanted sour flavor of creme fraiche.
cooking.stackexchange.com/users/1393/Michael Natkin
For protection against heat, you can stabalize the milk with starches.
(eg, a white sauce made with milk won't curdle, even if you boil it).
I unfortunately don't know how much you'd have to add, and of course, it'll end up affecting the texture of your soup).
For soups, you're likely better off making a corn starch/milk slurry before adding the milk, so you don't have to cook it like you would flour to get rid of the raw flour taste.
You may also want to consider the heat that you're cooking the soup at -- you likely want a low simmer, not a full boil.
cooking.stackexchange.com/users/67/Joe
you can also possibly experiment with creme fraiche, which won't curdle, but the flavor is different (nuttier, but not as sour as sour cream) than milk, but at least you won't have the curdling problem! :)
cooking.stackexchange.com/users/1415/franko
There are numerous chicken stock based soups that also have milk as an ingredient  and the milk always seems to curdle - how can I prevent this?
If I'm using milk as an ingredient in soup, how can I prevent it from curdling?
cooking.stackexchange.com
<soup><milk>
cooking.stackexchange.com/questions/4094
cooking.stackexchange.com/users/177/AttilaNYC
Try heating the milk to 90 degrees C before adding it to the soup, and use milk with a higher fat content. Cream or crème fraiche should resist curdling better than milk. Use a high fat milk and heat it before adding to the rest.
Try heating the milk before you add it in. Higher fat milk is less likely to curdle.