Air flow was key. Tried to get a good cross breeze from one end of the house to the other (unless the breeze is also hot). At night, we’d use a box fan in the window or door to get all the hot air out. If you can block your exterior walls/windows from getting direct sunlight, that should help. Curtains work (or you could also just place objects or plants in front to block the sunlight). Alternatively, you could install one of those large roll up sun shades for a more permanent solution. Did that for a few windows that get a lot of sun and made a big difference.

From personal experience, the following made a difference (but some will have a lot higher cost):

1. Replacing the insulation in the roof/attic. You could also add radiant barriers, but insulation is probably a better bang for your buck.
2. Insulating the exterior walls (drill and fill). Our walls used to be hot to the touch in the summer before this.
3. Double pane windows
4. Seal any air gaps or holes. Expanding foam is pretty good at this.

From what others have told me:

1. Installing a whole house attic fan to evacuate hot air at the end of the day. My neighbor did this and swears by it.
2. Painting your house a lighter color. Can’t say how effective but makes sense intuitively.

Stuff that only sort of worked:

1. Swamp coolers. Works ok in a pinch, but your house will feel humid like the tropics. Would personally skip.
2. Portable AC. The exhaust hose gets hot so it’s not as efficient as an external AC or window unit. But it could help if you’re directly in front of the cold air vent.