If you downsize a photo a great deal that will happen, it has to do with the mathmatics involved in reducing the number of pixels used to represent the picture. In the graphics biz the trick is to reduce the photo a little, then run a sharpen filter. This is done repeatedly until the desires size is reached.
The most consistant way I\'ve found is a 10% reduction, then a sharpen (not too much, just enough to notice). Do this until you reach final size and it should keep your detail and sharpness.
Good luck!