It seems to me that's very tricky to get people engaged in a SE site outside of the one they initially joined. One incentive might be to introduce network-wide badges.
These might include, for example:
- Having over 1000 reputation on 5+ SE sites
- Have a question with 5+ upvotes on 5+ SE sites
- Have an answer with 10+ upvotes on 5+ SE sites
I'd personally like to see this, but I think it might be good for SE as a whole, too.