What everybody said about ISTps and clothes/hairstyle was interesting. I hadn't thought of that. But of course Si doesn't necessarily equal a preoccupation with pleasing others, it's more to do with pleasing oneself. It makes sense now.
Interestingly enough, it's made me remember that "my" ISTps aren't too preoccupied with looks either. One was rather sloppy as a boy, but once he caught on that girls like guys to take at least SOME trouble over their appearance, he quickly changed his ways. Both sort of resist fashion. They'll wear stuff because it's comfortable, but in general it's more like "never change a winning team". Jeans are comfortable, therefore they wear jeans. End of story. No cargo pants or other freaky stuff.
Not sure you can change that. I personally think nothing can change an ISTp who doesn't want to be changed. An ISTp who is digging his heels in is absolutely unshiftable. They hate being pressured. It's possible to make suggestions, though... no, not even that... to state one's own subjective impressions and ask him what he thinks of it.
I'm beginning to think that this might be the key thing, the "what do you think" bit. Other types seem to value it if you take charge and say "this is what's wrong, this is what we should do", but the ISTps I know seem to detest it. It seems to feel to them like you're taking charge, crowding them out of the picture, not leaving them any freedom to act and have their own opinions. Could be the "well, it's HER thing now, I can't interfere" thing. Not sure.
I'm starting to simply present my own impressions and asking them for their input. "This is the impression I got, and my first impulse would be to go and do this-and-that, what do you think?" Or anything else to make sure they know I'm not actually telling them what to do, I'm simply making up my mind, thinking aloud, trying to come to a conclusion. That way they know there's still space enough for them to act in. Perhaps that ties in with static vs. dynamic thinking? ISTps rarely think aloud, so once you state your opinion, perhaps it's natural for them to presume that this opinion is unshiftable and they can't change it anyway. Not sure. Anyway, that sort of strategy seems to work; the ISTp seems to understand me a little better now and I avoid pissing him off.
Anyway, an example... let's say I want my ISTp to try on a pair of cargo pants because I think that would suit them.
WHAT NOT TO DO:
-- criticism ("you're always wearing jeans, you're so old-fashioned")
-- giving orders ("next time we go shopping I want you to try on something new")
-- sounding definite and bossy
WHAT TO DO: make sure he knows that nothing is hewn in stone. No pressure. Lots of freedom. In the end, Mr. sensible ISTp good-humouredly follows Ms. curious ENFp to the stores and tries on something new, because - like the obliging guy he is - he knows Ms. ENFp has got this bee in her bonnet about cargo pants and can't rest before she knows how he'd look in them. Ms. ENFp also points to an obnoxiously pink shirt or to a silly hat, "hey, that would look nice on you", so Mr. ISTp can grunt something drily ironic. We both enjoy ourselves and it MIGHT end with Mr. ISTp going, "hey, these pants aren't as bad as I thought. Do they have those in khaki?"
I'm probably going all Te-Hidden-Agenda now and sounding more sure of myself than I actually am... So I'll add that this is just a guess of mine, and something that seems to work with one ISTp I know of and I think it might probably also work with the other one. "Work" sounds like manipulation, but it's not; it's simply about avoiding to trigger the ISTp's I'm being pressured emergency shutdown mechanism.