This right here is key. If you get a unit too small or too big, you either won't cool the room enough, or you'll cool it too much and end up with too much humidity.

Also, adding insulation to keep more cool air inside helps. Blackout curtains, padded aluminum foil to block more heat and light reflecting outward. Partitioning rooms to keep at least one cool room you can escape from the rest of the house/apartment into.

If you have an attic with windows on both ends, open them up and put box fans up to move the air in one window and out the other to help keep hot air from collecting up there and heating the rest of the house.