I have 2000 notes in Evernote, but I've just left them there and mostly ignored them. It would take tens of hours to review them all. The whiteboard is just for thinking about new ideas. When I'm done, I clear it. To be even more explicit:
1. Whiteboard is empty
2. I write for 20 minutes
3. I review to see what's worth saving, and try to compress it into just 4-5 words.
4. Save 4-5 words in a personal list (I have another custom-made app for this.)
5. Delete all the text on the whiteboard (CTRL + A then backspace)
(then repeat)