losing sharpness

[Greensleeves]

Hi
I was wondering if anyone here was using cs4 to resize their photos for their mugs. I am having trouble keeping the sharpness in my photos when i resize them for my printer. The image i use is big but i use cs4 to resize them. both by image resize within cs4 and by click and drag. When i have resized the picture and look closer on my moniter it looks blocky where there was a very good sharp defined picture before i resized it. I was wondering was anyone here having the same problem and how did they get around it? Thanks
Patrick