Yay for your signature hair, Una! I think everyone on YLF loves that cut almost as much as you do. It definitely is iconic!
I guess I'd call my curls a constant if not a signature. My hairstyle changes pretty drastically every few years... but I never knew how much I depended on the wave in it until a particularly, uh, 'enterprising'? thick-skinned? stylist straightened my hair out for a magazine photo shoot. I have never hated my appearance and felt as alienated in my own body ever before, nor ever since. And this despite envying many friends their stick-straight waterfall of hair and lamenting my nonstop tangles!
Likewise my angled, mismatched, not very 'feminine' eyebrows. I get majorly ticked off when someone at the beauty salon tries to 'curve' them 'prettily'.
The other bit of 'look' that has been a constant for me for now 18 years is the single earring I wear in pierced right upper lobe (the left counterpart closed up almost immediately and rejected the piercing) --- what I wear varies from pearl studs to silver barbel or ring, nothing unconventional enough to call a signature; but the asymmetry is itself unconventional and was even more so when I first got it (single pierced ears are more common now than they were in my youth). Tangential: Actually, I've always enjoyed ear jewellery asymmetry, I guess. Even the standard love piercings used to get mismatched studs often in my youth, less so now perhaps (largely as I've been having trouble finding 'my style' studs of late, not sure why).
Lipstick, in terms of make-up, is the one thing I'm hardly ever without. Eye make-up I do often forgo.
Clothes --- I'm not sure I have a signature look there, unless it is midi-length skirts and wide-ish leg pants, which I've never been without. But they aren't quite constants.