Do not use this repeatedly for the same issue: When discussing a very serious issue or a problem you have been over with this person before, this technique is not effective and a more direct approach is required.
Don't give only positive feedback during this process : If you only give compliments during sandwich sessions, people will start to cringe when you start saying good things about them and wonder what they've done wrong.
Don't be condescending: You are trying to change behavior. Don't take an overtly superior attitude; don't be indignant; don't be pretentious... that is guaranteed to damage the communication effort.
Make the compliments sincere and related: People will notice if you're patronizing them - the intent will be obvious and the technique has a lower probability of success.
Avoid blame: What you're doing is pointing out something that needs to change. How you got here isn't important. What's important is where you are now, where you're going, and how you'll get there. As a whole, the conversation must be positive. Yes, there will be a negative part, but the two positives will outweigh that. Leave your listener with an upbeat attitude and you'll get the results you want.