Use sucralose (Splenda), Ace-K, saccharin, or stevia, preferably in that order. Splenda is not metabolised for the most part and is very close to eating air. The only one to avoid is Aspartame, as many people get kicked out of ketosis on it. Be careful not to overload on sweeteners however as your body expects sugar to enter the blood stream, and even though it doesn't with fake sugar your body releases some insulin anyhow in expectation. It shouldn't be an issue unless you pour it on.
Also, kool aid is a BAD idea. it has citric acid in it which can be worse than aspartame, and then amount of sweetener used with kool aid is large (may cause insulin spike). Aspartame is very easy to avoid, but citric acid is almost impossible to avoid in beverages. even flavored water usually has it in it. Some people are more sensative than others. Diet Dr. Pepper is about the only pop that doesnt have any (but it does have aspartame).
Hope this helps...