Unless you are in the first bloom of youth then I think it’s the basics that take some effort that make the difference but as a PP says a routine really helps
Hair - As Hilary says, hair matters. It needs to be groomed (even if it’s tousled groomed) and shaped. Regular cuts. Experiment with what products/routine works for you and them do it.
Teeth - white ish and even ish. Save up to get fixed if needed.
Eyebrows shape your face. Get them shaped regularly (I think threading is best) and colour in as needed.
Nails. No need to be ‘done’ but keep them tidy and even.
Good glasses if you wear them.
Small shiny earrings help like up your face and create a generally groomed impression.
A decent bag. Doesn’t need to be fancy.
A decent coat.
posture as PP says. Yoga ladies always seem to have good posture and movement.
Smile. Poss yoga also helps with this? Tell me if you find out.
I think Mary Janes over the age of 6 are very wrong, but there are may more comfortable and flat shoes than ever - I like Fly London and Fit Flop have some good basics