When you are resizing images some blurriness will happen. You just won’t be able to shrink an image down without some loss of quality.
Some comments though, assuming you are resizing these thumbnails in Photoshop:
— when you resize them, use Image > Image Size, and down at the bottom of that Image Size popup, there will be a dropdown with several options ("Bicubic" is the default). When making resizes, select "Bicubic Sharper" which will help improve clarity.
— if you want to rotate the thumbnails a little to add interest, remember to rotate them first when they are large images, and then resize them down. If you wait to rotate the image until after you have resized it, there will be a significant loss of quality.
— Finally, check your quality settings when you are using Save For Web… Perhaps you can also increase your quality setting slightly, and that will improve things as well.