Dips of levels 1, 2, and 4 for fighters. Dips of 4 levels for any 3/4 BAB class, because remember, you only lose out on BAB at levels 1, 5, 9, etc. Dips of 2 levels in any 1/2 BAB class for the same reason.
However, with all the prcs there are in the various splatbooks, dips are a lot siller...