This is a fairly common issue when resizing images, but with Photoshop there's a solution.

This method will only work in Photoshop CS and CS2. These versions are more powerful in a lot of ways, so if you don't have them I'd recommend that you invest in a copy as you may find that you will need the more advanced features.

To reduce the size of a picture without losing too much resolution go to Image> Image Size. Click on Resample Image and then pick Bicubic Sharper from the options. This will produce the best results when resizing an image as it significantly reduces the blurring of the image when you change its size.

When you want to increase the size of an image, but want to minimize the resolution loss choose Bicubic Smoother.

This method is extremely effective in keeping the sharpness of the image. You can set this as the default to make things easy for you by going to Preferences> General where you will see Image Interpolation. Select Bicubic Sharper as the default choice and it will be set as the default from now on.

It is important not to perform multiple resizing on an image. To experiment with image size, use a copy of the original image as many resizings will reduce the resolution of the image more than is needed. Use the copy to find out what size you want then use the dimensions on the original to keep resizing to a minimum on the image you will finally use.