For me, the best way to understand topic sentences was to see really good examples in action. You should also talk about the different kinds of TS -- a TS that presents an already mentioned point for further inspection versus the "change of direction" TS ("Another way that the author demonstrates this is through..."). I think mastering the latter is particularly important. My essays always were criticized as making abrupt transitions or jumping from point to point until I mastered that type of TS.

[EDIT] Oh, another thing I just remembered. It really helped me when a teacher told me to write the whole essay first, then add in the TSs at the end. This can also help clear up more meta- structural problems with the overall essay. Of course, this is only a reasonable approach if the writing is being done on a computer, but perhaps you could just have them leave a few lines blank at the start of each 'graf to fill in a TS later (Jackie is right about the "put it first for now, get tricky later" theory).