Apartment on the top floor gets very hot - cheap, eco friendly ways to keep the place cool?
Adding insulation to your ceiling is probably the ultimate answer, but it may be expensive and disruptive.  The master bedroom in our last house was below a flat roof, and I found the following measures made for more comfortable sleeping:

1) Since the cold air from air conditioners tends to sink, use fans to blow it back up to the ceiling.  From there it will descend and you will feel it more than if it were just all pooling on the floor.  Placing a fan near the A/C source may also help draw a greater flow of cooled air.

2) If you can open windows and the air is cool outside, use fans at the base of the window to blow air upwards.  Same principle as before.

3) Humidity is as much your enemy as the heat.  Although A/C is supposed to dehumidify, I've found that running a supplementary dehumidifier can help.  You'd be surprised how much water you can draw out of the air.  No point doing this while the windows are open, though.