There is the quick way and then there is the measurable Recycle program way where you track leads waking from the dead and build into the sales process complete with alerts and lead conversions.
The Quick Way To get up and running quickly, build some kind of inactive smart list that you will use over all your alert campaigns. Your inactive list could be made up of items such as "Not Visited Web Page", "Not Clicked on Link", etc over X amount of days.Then use that "Member of Smart " smart list as part of any of your triggered alert campaigns.
Note that Marketo only tracks activity for a certain amount of time for Smart lists (I think 90 days). If you want to do something longer, you may need to do something a little different like "Add to List."
The Complete Recycle Program This process involves elevating leads base on scores, changing statuses to bring into Sale's queues, etc. There is a 15 min preso on how to do a Recycle Program from the 2013 User summit. Skip to minute 30. Any questions, send me an email.
I also looked at creating a campaign using visiting website adn fills out form as triggers in the past 30 days. Then those are filtered by lead creation date or smart list.
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