Am over 50, tall, but not as thin as once I was, alas. Agree with other Mumsnetters that good bra is essential. Also, I think it's important to find the shoe heel height that you find comfortable with and suits you. Then you can look for the most flattering styles without worry. And not fall over...
I have bright white hair/pale skin/ light (green) eyes and I find that black, navy or dark grey basics plus either subtle (silver grey, bottle green, petrol blue) or bright (emerald green, electric blue) jackets and scarves are my friends. The neutrals are good disguise, while the colourful elements distract from the bits you'd like to hide while you are losing weight.
People with different colourings - from rich and warm to pale and ethereal - will find, I am sure, their own basics and highlights. Think hard about what colours/shades really suit you and don't bother with anything else. Ditto shapes - are you elegantly straight up and down, or hourglass? Dress accordingly.
Don't go overboard for 'statement jewellery', but the odd dramatic piece can be helpful.
Sounds daft after all the above, but I really, really, don't want to sound prescriptive. However, IMO subtle and luxe and discreet are far more flattering than anything too obvious.