The tap to clear thing actually works, but only for the app in question. To do it, say, in the iBooks app, simply tap any one of the icons at the bottom of the home page 10x relatively quickly. After the 10th tap the screen will blank for just a second signifying that it is clearing the cache. You can repeat this in any Apple created app that has a home page configuration with buttons at the bottom, like iBooks or the App Store (not sure if there are others).
The movie thing does work, too, but you do actually have to select "buy". The key is to pick a movie that is larger than the amount of space you have remaining. If you do this, you will not actually buy the movie, as the system will clear the cache trying to buy it and then report back to you that the movie is too large to purchase. The key is finding a large enough movie, because if you don't, you might accidentally clear enough cache and actually buy the movie and have it install. A recommended movie is "The Two Towers" from the Lord of the Rings trilogy. It's over 8GB, so unless you have a large capacity device that had a huge cache built up, you should be OK.