Tom,
I have made it a general rule to mark my leather prior to any tooling. For two reasons.....first, I can tool around my mark. Secondly, there is nothing worse than tooling hours on a project and finishing it with a bad stamp imprint.
As Badger mentioned....case your leather and let it dry to the touch. Too wet and the imprint will look "mushy". There are several ways to strike your stamp. On smaller stamps (1-1.5") I use a mallet. Just one solid strike. Hold the stamp firmly to the leather and DO NOT allow it to jump. This will result in a nice clear imprint. It's a good idea to practice with your stamp on scrap leather. ALSO, make sure you are using a piece of granite, marble, or solid tooling surface to strike your leather on.
Good luck,
Daryl