Put up awnings over your exterior windows, place translucent bright colored/white films over windows, get curtains with a white outward facing side.

In the same week after doing the above, the interior temp of my living room was almost 10 degrees less, on the warmer latter half of the week. And I live in what is essentially a trailer cemented to the foundation, so insulation isn't great.

A lot of people say get blackout curtains, but it's important that they have a light colored exterior to keep from absorbing heat and radiating it to the inside. The two windows in my bedroom are total black on one, and white/red in the other. The black one blocks out all light, but is noticeably warm next to the window. The white/red one is much cooler, even when in full sun.