It may seem elusive but the best practices for combining Navigation Controller with Tab Bar Controller are very well covered by the Apple docs. Furthermore, even the instructions on "HOW TO DO IT" when using NIB files versus doing so programmatically are available too.
Having gone through this process myself, I must admit that I made various misinterpretations with the NIB file related instructions, which wasted way too much of my time. I even reached a point where I had discovered a new-ish but probably improper way of making things function. On a whim I luckily went through the whole process again ... only to this time (thankfully) discover the correct approach.
So as a supplement, I've made a screencast that aims to close this gap for any other novices. Enjoy!
While my screencast deals heavily with the NIB file based approach, the 7th iTunes University lecture by Paul Hegarty starting at 29:50, demonstrates how to do so programmatically.