I don't know if all leather treatments call for this, but I have had good luck letting the seats heat up a bit (park in the sun or put a light shining on them) to help open the pores of the leather up. That way when you put your leather treatment on, the leather really soaks it up.
"The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong, it usually turns out to be impossible to get at or repair."
-Douglas Adams, Mostly Harmless