We've used tinfoil before, but it doesn't look that tidy. When we moved into our new house in January though, I really didn't care what the neighbours thought, and even covered the windows of our "camp-out" room with plain old cardboard.
At the moment we have curtains made with a blackout lining (thanks Grandma!), but even that wasn't enough. So I went to Office Depot and bought pieces of black foamboard, like kids use for their school projects and presentations, and carefully cut that to the size of our window panes. It was a little finicky, but does block out most of the sunlight. The fit is so snug that they stay in by themselves, but I do have a couple of pieces of reinforcing sellotape, as a couple of pieces fell down mid-nap. (I heard them fall on the monitor, but surprisingly DS didn't awake!)
The Brits need to speak to their Mums and Grandmas. What did they do during the blitz? They had to black out their windows completely then!
Katriona