I'm a series collector primarily. I have 4 series going on: Peace dollars, $1 silver certificates, barber dimes, and V nickels. In the first two series, I only have key coins/notes left. In the others, I have tons of holes. How does this relate to my shopping routine? Well, when I go to shows, I have $X that I want to spend. If $X means I can fill one of the key holes in the first two series, I'll make that my priority. If I don't see one of those rare coins/notes at the show, then I'll drop down to the easier series and fill as many holes as I can.
In short - I love the hunt, so I'm pretty much only adding to my collection when I go to shows which are far and few between.