overexposure of sky, what to do

When you look at a scene with the naked eye you need to understand how it is lit. A modern camera sensor can only capture about 4 - 5 stops of exposure while the human eye can process about 10 stops of exposure. Therefore we are always trying to find scenes that have decent balance. Good shadows and highlights, with a decent range of neutral lighting. Looking at a histogram you would see this as a scene with sharp curves peaking at about 80% on the left (dark side) and right (light side) of the histogram with the middle being at about 18%. The problem is that this is almost always not the case so we have to make some compromises. When there is an excessive shift to the left or right side of the histogram we will almost always have exposure problems. The key is understanding where the most balanced lighting point in your scene is in these circumstances and using something like AEL to lock your exposure on this spot and then taking your shot. You will still end up with lost details in shadows or overblown highlights even when doing this, but you should be able to fix these in post processing.

You may also want to experiment with bracketing at 1/3 stops. This will shoot an exposure 1/3 of a stop down, 0 and 1/3 of stop above your metered exposure reading providing you three photos to evaluate during post processing to try and drive out your best results. I find 1/3 of stop down (especially in bright scenes) gives me the best opportunity to get the best detail and lighting out of landscapes.

Here is a good introduction on how to read a histogram; you should really invest some time understanding how this useful tool works:

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

Cheers