Until now I've used a bicubic resizing of the images. If you don't know what does "bicubic" means, suffice to say that it makes smaller images smooth. It makes them blury, in fact.
So know I apply a Lanczos filter to make the resize, couple with a little local contrast improvement filter.
Both effects are quite subtle, and of course your original image remains untouched. If you see your images a bit more sharp than before, well, you know now why.
Hope you like it!