20. Have something cool to work on. Top talent doesn't want to rewrite your java accounting system (again).
What applications the team is going to work on is definitely a factor for how attractive a job is, but it really isn't something you can easily change just to get new people in.
Bring someone in to mentor 2 or 3 others. Have them act as a software toolsmith for the others. Also have in the plan the transition to the rest of the group.
Software tool-smithing is often very interesting work, which lets your uber-programmer stay an arms length away from the boring corporate programming. However, you still get to benefit from his high-level skill. Also, the transition to others, if done right, focus his intellect in the direction of "how to make things understandable."