Jeff has the right idea.
There are several threads on score decay and inflation - just do a quick search.
Usually with a decay score you can run this daily, but leads only flow through every 30 days:
Smart List: if no activity (or score hasn't changed) in past 30 days (usually I use Not Visited Web Page, Not Opened Email, Not Clicked in Email, etc...)
Flow: Score IS -20 (or more or less)
Also have several negative scoring campaigns for email bounced, unsubscribed, and other bad behaviors.
This might help too
http://www.slideshare.net/jdavidhill/lead-scoring-with-marketo