I'm sure we've all had it at some point or another. We play against an army or see a new model and we think "Oh, man I want that."
I know for me, this happens all the time. I have the unfortunate tendency (due to being a college student in the off-season) to make an army list (or two or three) a day using different codices. While I do think it's fun and it helps me understand what to expect from each army, I always get that desire to build or collect that specific army.
Three lists that I've done recently are a Sisters of Battle army (triple Exorcist & Callidus Assassin :D), a Thousand Sons army and a Space Marine Scout list and have seriously had to resist the urge to start buying (even though my Salamanders aren't even near completion).
Fortunately for me (or if you'd prefer, unfortunately), I have little to no money to spend on Warhammer at the time and that's my deterrent from dropping into model fever.
What do you guys do to resist the urge to buy new models?