No to matching shoes and bag. A big NO. Matchy match is VERY dated. Only acceptable if you're over 80 and on a cruise.
Play on volume carefully - if wearing boyfriend jeans wear a fitted top/T-shirt, if wearing leggings wear a loose-fitting top, etc. The only time I break this rule is in the gym when everything is skintight (so it doesn't get caught in equipment etc).
No to heels if you can't walk well in them (nothing worse than a woman who looks like she's walking - badly - on stilts). Actually I steer clear of heels for anything other than black tie. Flats/trainers is a much more modern look.
Legs out or cleavage out but never both. I always go for legs as I think cleavage looks a bit too tacky.
Don't wear gold and silver jewellery together. Pick one and go with it. And don't go OTT. Statement earrings OR statement necklace etc but not both.
Edit your look. Don't over-accessorise. Pick one or two good pieces (scarf, necklace, earrings, sunglasses etc) and make that your focal point for your outfit.
Don't be afraid of colour. Just make sure if flatters your skintone and colouring. I avoid pastels as they wash me out. Strong colours work best for me.
Think about texture. I have to like how something feels against my skin to be comfortable. But also, you can dress head to toe in the same colour but mix the look up by using different textured fabrics.
Don't be a slave to fashion. Some people just slavishly follow what's 'in' regardless of whether it suits them or whether it flatters them. Buy what you actually LIKE and what actually that flatters you. For example, the current fashion for long frilly dresses is one I avoid. I think they look hideous (even on models) and I know they will look horrendous on me as I'm tall and slim but very gym-honed. A flash of biceps from under a frilly sleeve is not a good look. 
Polish your shoes before leaving home. I can't stand scuffed or dirty shoes. Attention to detail counts. Which reminds me...polish your jewellery too and it will look so much better.