Once you have figured out making your own yogurt (which is very easy by the way. You'll find a great picture tutorial here.) you will want to experiment with this trick. Line a colander with a double thickness of white paper towel. You can also use cheese cloth or muslin fabric. Place a bowl under the colander to catch the whey that will drain through.
For a sour cream substitute, fill the colander with plain yogurt and allow it to drain 30-45 minutes. It all depends on how thick you would like it. Many casseroles call for sour cream and milk. When this is the case, I actually just use the plain yogurt and don't even drain it. I then omit the milk or only add just a little to get the consistency the casserole needs.
For a cream cheese substitute, allow the yogurt to drain for a few hours or overnight. The yogurt will reduce in half if you leave it long enough. You can then use this in recipes calling for cream cheese. I have not yet tried a baked cheesecake with this substitute, but the no-bake variety works just great! I also use it in vegetable dips or fruit spreads as well as casseroles or mashed potato recipes that call for cream cheese. I have had success every time. You may want to search online for recipes using "yogurt cheese" that is the technical term for this substitute.
This substitute is completely fat free and is much less expensive to make using your powdered milk. So you can feel extra provident by using your powdered milk and saving money at the same time.
What to do with the whey that you drain off ? This is a high protein liquid that works great to add to bread or muffins in place of the water that is called for. Freeze it if you'd like or just store it in the fridge for a few days and work it into the next recipe that you can.