I was caught out every year by summer, so last year I decided to sort it. I bought:
- some decent sunglasses
- several pairs of sandals, ranging from practical and sturdy to high gold wedges, so that I have pairs for walking miles in, pretty but slightly flimsy flat ones for wearing round the house, and more dressy/delicate pairs when I don't need to walk as far. I bought most of mine either from independent shoe shops selling European brands built to last, or from LK Bennett for the pretty but impractical ones.
- some knee-length loose cotton A-line skirts, mostly from Fat Face though I also have one or two from New Look and White Stuff, that sit flatteringly on the hips so that I still have a waist.
- some semi-fitted short- or cap-sleeved shirts and blouses in 100% cotton or linen (I am too blessed in the breast department to look good in T-shirts as I need buttons to break them up visually and stop them looking like a shelf, but they'd also work with T-shirts for someone smaller-chested). Also a couple of loose long-sleeved shirts in soft grey/blue from Mint Velvet that I wear with the sleeves rolled up to the elbow.
- some summer trousers. For me this was Boden Richmond cropped trousers in navy, Boden Rachel chinos in camel/beige that can be worn full-length or rolled up to mid-calf, and a pair of straight-leg M&S jeans to be worn rolled up to mid-calf since my normal Topshop skinnies are far too tight to roll up.
- a couple of light cotton knee-length dresses from Hobbs in navy or navy print that I wear with tights in winter and bare legs in summer - very versatile
- some lightweight summer cardigans in navy, camel, cream and white to throw over the top on cooler days or in the evenings
- I already had a couple of jersey FCUK dresses in prints that look quite summery, but I'm not comfortable with above-the-knee dresses except in tights, so I also bought some navy leggings and wear those with the dresses and sandals on in-between days, which we get quite a lot of in the UK
- I also bought a linen jacket in a charity shop and two light trench coats in navy and camel for cooler days or evenings
I avoid panic-buying anything that just does not suit me: e.g. I am a size 10 but short and very curvy and look awful in shorts, vests or sleeveless tops of any description, so I don't bother: I still want to look good even if it's hot so stick to cropped trousers or skirts and dresses and cap sleeves. Another thing that really helped was to find a colour combination that a) suits me and b) looks suitably summery since I realised that so much of my wardrobe was co-ordinated around black so that I looked ridiculous as soon as the sun came out. In my case this new colour combination was navy and white. This means that most of my summer clothes go with each other because most of them in variations of navy, navy prints, white, cream, ivory, or beige/nude, but with loads of small variations so that I don't think it's obvious. Navy also has the advantage of being a year-round colour so I can keep wearing a lot of my dresses and skirts with navy or black tights and brogues or boots as summer moves into autumn.
Good luck! I'm still not 100% happy with my summer wardrobe, but the emergence of the sun doesn't cause me total panic any more like it did year after year.