Unfortunately I don't have enough time to manage (sell / update) my advertising space on my network of websites as I am working on developing / expanding the network.

I wanted to get someone else to do this for me and share a % of the income but I don't know how to manage it or make it fair.

Advertising will be sold in slots of 1 month, 3, 6 and 12 months.

How could this be managed if for example someone does this for 1 month, gets 3 advertisers at 3 months but then stops working on helping advertisers, getting new advertisers etc. - How do I make this fair for both sides?

Thanks for any help