It is a hard concept for them to understand. After moving on from the manipulatives to the pencil/paper this is how I do it.
I would say: "Can we do 7 take away 9?" If I have 7 apples could I give you 9 of them?
Kids say: No
I say then we have to regroup. I'm going to go up to my next door neighbor and say " Mr. 5 will you please give me 1 number? Mr. 5 says sure.. Then I cross out the 5 and write a four.. I put the number he gave me (1) next to the 7.
Then I do the problem. This really helps them because they think it is funny. I can ask them again and again what we have to do and they will say, borrow from Mr. ....
I hope this helps. It is helpful once they get going with the regrouping to throw in some problems that don't need regrouping, because they get confused and forget that they don't need to regroup.