My INTJ friend and I have a great strategy. We ask each other! Seriously, though, INTJs like practical things, they have no use for pretty baubles. In my experience, books from their field of expertise are highly appreciated (or else fiction that you know they like and don't yet have). Practicality, that's the one.
Your best bet is finding out what they meant to buy for themselves and then getting it before they have the chance to. Someone said that before, but yeah, that's about right.