Good regular hair cut (and colour). If you are on a limited budget, put as large a percentage of grooming budget into hair as you possibly can as it is the first thing everyone sees, you can't really disguise it. (Wish I could take my own advice on this - I absolutely loathe being pinned to a chair at the hairdresser's - almost phobic about it - so get in and out as fast as poss and use colour from a pkt!!)
Decent quality shoes and handbags. Shoes should be polished and not down at heel.
And well thought out accessories such as scarves and jewellery (these don't have to be expensive)
My own personal one - (but boring/classic dresser so may not appeal) is always have a well cut navy blazer/jacket that fits. Can dress up or down. Invaluable capsule wardrobe item.
And if you wear black always make sure you are well groomed, with polished shoes, brushed jacket shoulders and lapels, no frayed hems or missing buttons, or it looks tacky
Lay out clothes for the next day the night before together with all accessories, shoes, underwear jewellery, bag etc ...that way you can spot any missing buttons or tears, and have time to make small adjustments
Have access to a good-sized mirror in decent light
Invest in 3 or 4 good quality, well-fitting bras (at least two of which should be in nude colour)
Depending on life-style, always put together two "interview/wedding/christening/funeral/posh going out outfits" together per year at the beginning of the season. So around about now, put together posh outfit for winter, again with shoes/boots/tights, jewellery, bag etc etc and at end of March/beginning April make sure you have the equiv prepared for summer.
Every so often (every five years or so) go to department store and get free make-up consultation as skin tone gets paler as one ages.
Ring-fence one hour minimum per week to personal grooming: manicure, pedicure, face-pack, de-fluffing, eyebrows, fake tan etc etc (if I can be bothered, I usually do this late Sunday afternoon)