Difficult question, hmm. I'm a weird person - I simultaneously put much thought and little effort into my appearance, so anything that a) I'm already disinclined to do, or b) would demand more effort on my part... would be ignored.
There's also the fact that such a suggestion could easily become a big ol' hint that my S.O. doesn't know me that well. Heels, make-up, etc. aren't just an aesthetic choice, they're an identity thing as well. Asking me to start that stuff would be like telling me, "I see you differently from how you see yourself", which would be a little uncomfortable. I'm not a tomboy, not at all, but eh... okay. Getting into that whole gender/identity/looks discussion would be pretty off-topic.
Parties are usually an exception because they're made to experiment with looks, and I try not to take them too seriously anyway. I'm more inclined to wear something girly or sexy there, and to take suggestions.
Changes in colour or smaller style stuff, eh. If there's a good idea there, then why not hear it? I probably wouldn't follow it if it's too effort-demanding, though.
Hair... *snicker* I'm conditioned to ignore 80% of all hair comments - ever - because they're always "noooo why did you cut off your long hair, it was so pretty!". However, I'm always looking for suggestions and ideas for things to do with it, so I'd probably welcome any comment that accepts that my hair won't go below shoulder level again.
Truthfully, I'd LOVE my boyfriend to suggest really short hair so I'd have some encouragement to actually do it, but that's not gonna happen. Ever. Stupid "girls should have long hair" paradigm.
On the other hand, he won't grow his hair out for me either, so I guess we're even.
I actually have influenced my boyfriend's looks a little. When I met him, he liked to use hair gel, which had the unfortunate consequence that I wouldn't touch his hair at all when he used it. Apparantly hair snuggles are more important to him than looks. Whaddya know?
TL;DR: I'd resent most "girlifying" suggestions (including heels and make-up) and ignore any "grow out your hair!" cries.
Any other suggestions in hair or clothing style would be welcomed, but not necessarily followed. I trust my own judgment too much to just go with something because my S.O. requests it.
Sorry about the long, rambly post.
This. A thousand times this.