We've done a number of summer and August visits.
1. First, I don't personally find the heat to be unbearable, but it usually helps to take a mid-day break, when the crowds and heat peak. If you are concerned about heat, then mornings and after dark are the best times of day. There is a short period each day where the humidity can be a bit intense. I'm not sure how to say, but starting mid-day is the worst. That is the most dramaitc. At 8am, the heat is not bad, and it builds gradually. It also helps if you can spend time out of air conditioning before going to WDW.
2. You can somewhat stack LL, but that still requires someone to be awake to book them.
3. WDW considers August to be "Halloween" season, so one option to consider is to book a Halloween party on one of your days, maybe instead of a regular park ticket. If you are staying at a deluxe, try to use the PM extra hours, assuming WDW is still offering them.
4. Several folks have said there is afternoon rain. When it rains in August, it comes down in buckets. Not only that, but VERY often it is a thunderstorm. Thunder = many rides stop running, all the pools close, animal attractions close, boats stop. Most of AK stops. Some days the rain lasts 15minutes. Other days, it lasts for hours. A day that starts at 4pm might = no coasters at all, no outdoor boats, and no animals. AK is limited to indoor shows, FoP, Na'vi, Dino and little else. That also means everyone in the park is vying for those few attractions, or taking shelter. August is also hurricane season.
5. Our strategy is: bring 2 pairs of shoes, both water tolerant. Quality walking sandals are a smart choice. Ecco and Teva are good brands, and shoes that look similar to those two. Sneakers can get very foul smelling, esp if they are not new and/or do not dry out. In the humidity, it can take days for them to dry. We try to wear sneakers only on dry days. Bathing suits also may take extra time to fully dry.
If rain odds are less than 50%, we carry cheap plastic ponchos. (any camping store for less than $3.) We try not to use them. I'm no rain whimp, but once they are opened, they can't be repacked, so we try not to use them.
IF rain odds are above 50% we carry jackets. Our favorites are something generally sold as a 'packable', sometimes called a packable parka. They are ones that fold down into one pocket. Very compact and lightweight. Uniqlo makes them, but the ones from Uniqlo are often NOT water tolerant enough for the heavy rain of WDW. Uniqlo makes a more expensive rain jacket, that I have not tried. Sometimes Gap has them in spring; sometimes LLBean has them. Though again LLBEan also carries $$ rain jackets. Other stores likely also have the packables in spring. We also have bought jackets that don't say they fold, but we were able to fold them into one pocket anyhow, they just don't zip/have a handle.
You might also want to keep rain in mind when choosing a purse/daybag/fanniepack, or backpack. We try to minimize what we carry, but make sure to carry blister bandaids and/or moleskin, or at least fabric bandaids that stick when wet in August. You can get bandaids at WDW's first aid, but if your feet get wet, having bandaids on hand is helpful. Walking all the way to first aid might be aways away, and a couple bandaids take up minimal space.
Last we DO make use of any PM hours. We don't pay for the Halloween parties, but we DO try to take advantage of the PM park hours. They can be great.
