I'm not slim but often have comments about how 'put together' I look.
I find the absolute easiest way is to wear dresses...I rarely wear anything else. I think it's simply that with a dress you don't have to consider pairings or what-goes-with-what...just throw on a dress.
I also don't wear very much jewellery....normally one piece only...either necklace OR earrings. Never both.
I actually wear a fair amount of make-up but the end result is the 'natural look' which is anything but natural.
When my nails are longer I keep them natural. I only wear nail varnish if my nails are short. Long and coloured starts to look too much.
I also agree with the crumpling comments...I don't EVER iron. Life is too short. So I just don't buy fabrics that crease....again this is much easier for dresses than trousers.
A decent coat is a must...I got mine with matching scarf from an LK Bennet sale. Still expensive but I will use it for years.
Interestingly I don't stick to neutrals...camel/grey/dusty pink make me look like I'm dying! You need a certain sort of complexion...I tend to wear the opposite....navy blue, red, cobalt blue, aubergine.
If I do something casual like walk the dogs, either wear a gilet or if necessary due to weather a Barbour. And wellies or knee length boots.
Sportswear and trainers should be kept at the gym. My pet hate is seeing people commuting to work on the train in a skirt and trainers! Ballet flats are the answer!