There are two ways of fixing this that I know of at the moment:
1) Edit sapphire/filesystem/GD.php and change the default quality from 75 to something higher (I find 90 works well). This will change the default quality for _all_ processed images. Of course this will get changed back when you upgrade SS.
2) Extend or decorate the Image class and use the setQuality method to change the quality of generated images. If you're inserting images into the content area of a page then you will need to decorate the Image class, while if you doing a relationship then can just extend it.
It would be much nicer to have an easier way of setting the quality though, even if it was just a constant in _config.php, as this is the most common complaint we have received from our customers.
Of course there may already be an easy way of doing this, but if there is then it's certainly well hidden :).