Great tips above!
Some of the cheaper card stock wicks the ink out into the fibers, and then your image will look blurry even if you stamped it well.
Is your inkpad juicy, but not too juicy? I like Versafine black ink for crisp images on very detailed images. (But you do want to wipe off the stamp pretty quickly to clean this.) I also find that it is easier to ink most stamps up by pouncing the inkpad down onto the stamp and then you can see if you got even coverage.
Padding under your cardstock helps. I use fun foam. Also, just making sure that you have a good solid surface to stamp on. A sturdy table, or countertop. Not a rickety card table.
For large background stamps or large images, I sometimes will have the rubber side up, and then lay paper over top of it and just evenly smooth over it with my hands.
I have had one or two stamps that were too brittle as they got old and those just didn’t work anymore.
All that being said, sometimes I just practice on scraps of copy paper till I get the feel of a certain stamp.