One of the first tips that I have found when researching this topic is to wear a column of colour. The obvious example is a black top with black trousers, however this isn't a good combination for most over 60s as black can be too harsh a colour. Another possibility, which is a bit more flattering for most women of a certain age, would be a navy jacket or cardigan worn with navy trousers or skirt plus a brightly coloured top in your best colours underneath. The general aim is for your eyes to be drawn downwards, creating an illusion of being taller than you are. It will of course be a bonus if this makes you look thinner at the same time.
It's clear that the model in the photo above is tall and slim anyway, but hopefully this illustrates the point that I am making. Wearing shoes that match the colour of your trousers also helps to elongate you.
Another tip is to wear outfits with vertical lines rather than horizontal ones. The trousers from Zara shown below are a good example, however I think a more subtle stripe would be better for petites, who can be overwhelmed by bold patterns. I own a pair of navy and white striped trousers that I usually wear during the summer months as they are cropped. The slight stripe means that they are more flattering than ordinary cropped trousers would be on a shorter person. As mentioned above, wearing cropped trousers with shoes of the same colour helps to create a longer line. Alternatively, they can be worn with shoes that match your skin colour.
I love stripes, but most of my striped garments are tops with horizontal stripes, which goes against all the rules for looking taller and slimmer. Take a look at the outfit below, as an example. I usually avoid monochrome outfits, so although I am wearing navy jeans, a navy and white striped top plus a navy and white floral scarf (scoring points for the column of colour), I have added a red belt and red shoes. These of course break up the slimming effect and make me look shorter! Does this really matter? I don't think so, as I enjoyed wearing this outfit and that's the most important thing in my view. At the end of the day I am a short person, so though it's good occasionally to create the illusion that I'm a bit taller, I'm not really deceiving anyone!