I suggest some type of an incentive program.
Lets say for example for every 8 hours that a node is active the node's operator would receive a certain amount of BTC.

Some things to consider:

• Node Operators would have to prove ownership, perhaps by installing a signed message on a HTTP server with the same IP address.
• It would require some development work to fully automate.
• Such a thing would undermine the privacy that Bitcoin seeks to create.