^^, i dont know about incentives so much.. but i do think you need to set goals and task levels.
Point in case, a lot of these VA services will get you a VA and you can use them for shit, but expect the same amount of work being done per day. Day 1 on a new task they build you 100 links, Day 2 is 100 links day 3 is 100 links, etc... on day 60 your still getting 100 links per day. Thats all fine and gravy, but your VA isnt doing anymore work, but you know they can.
You need to upfront say, I expect you to have this done in 3 days and have x amount of links/articles/KWs, etc