When the summer heat strikes, it often does so with little warning. And then you are in a situation where you need to deal with 90 or 100 degree temperatures when you are in the house. The first thing that we would say is that you may want to spend some time out of the house during the day. If you are usually at work, that is good. But even if you are not, maybe going to a public library or a coffee shop to do your work is a good choice. It will keep you cool and out of the house during the worst heat hours.
But what will you do the rest of the time? If you come home in the evening and night, the best thing you can do short of turning on your AC is to keep the windows open the moment it cools outside. Even on the hottest summer days, you will inevitably get temperatures that are closer to the mid 70s in the evening and night. That is when you need to get those windows open, and get some box fans that you can place in them. It will cool your entire house down very quickly, even the rooms that are upstairs.
If you struggle to sleep at night because it is too hot, consider changing your sheets to bamboo. It is much cooler and more comfortable to sleep in. You may also want to get a cooling pad for your pillow, while a cooling mattress pad can help too. If it is too unbearable even with these changes, think about soaking one set of sheets and putting it on top of the ones you normally use. Then turn on your ceiling fan and you will have a nice, cool feeling until you fall asleep!