The Ike Group has lots of information about Ike varieties and die clash marks.
One variety to look for is the 1972 Type 2. I've searched through thousands of Ikes and I've yet to find one. The 1972 types can be identified by looking closely at the Earth on the reverse.
Here's a page that shows examples of the three types.
The 1973 and 1973D Ikes were issued only in mint sets, not for circulation. I found my first 1973 last weekend.