Quite some time ago now we noticed that Spotlight were having a sale on blackout and thermal curtains, and decided to buy some for the living room to see if it helped the afternoon temperatures (the living room is at the west end of the house, and in summer time at around 4pm the temperature in there just skyrockets to the point the air conditioner simply can't keep up). We picked up some thermal curtains, enough for both windows, and put them up in winter time.
I thought perhaps it was a case of confirmation bias, but I definitely felt like having them shut overnight in the winter helped keep the temperature up... but the start of this summer sealed the deal. Even on a 42C day, the air conditioner was able to keep up and I'm fairly sure they help a significant amount.
We didn't think about it until Thursday, when another thought struck me. Duncan's been complaining that in the summer it's too bright in his room at bedtime (8pm), what if we got some for his room? We could get the thermal curtains as well and it might just help the temps as well. They happened to be reasonably priced again (after we just about left due to everything being $100+ a pair!) so we picked up two for his room and two for our bedroom, and I put them all up last night. What a difference in temperature!
If we end up buying a house, I'm going to have to get serious about decent thermal insulation because this is just silly - we spend so much on electricity (and who knows how much it adds to our carbon footprint) just trying to keep this stupid house comfortable when all we had to do is manage the sunlight better.