I think I'd have to start with the conditions at my current place of employment. There is a certain amount of inertia involved when you're talking about changing jobs, even if it's to a direct competitor located in the same building. The amount of force required to overcome the inertia can have two components: internal and external.
If I'm currently happy with my employment, then I am not capable of producing the internal force necessary to overcome the inertia. That means the external force has to overcome it all by itself. That's probably the most likely situation you find yourself in. That's hard, because the things you will have to do to make up for my lack of internal drive for leaving is high; as a typical example, you might have to offer at least a 50% salary increase, have a non-monetary draw (wow! it would be cool to work for XYZ!), have a location advantage, offer creative and unique benefits, or have a special work environment somehow (the smartest programmers in the world, all the admins are former playboy bunnies, etc, etc) - you might even need all the above. Different people will be attracted to different things.
If I'm currently unhappy with my employment, then it's much easier for you, because I'm supplying some of the force required to overcome the inertia. In this case, a 10% salary bump, or even a sizeable ($10K-$20K?) signing bonus might be enough. Telecommuting might be a plus for someone who is currently pulling out $1000 each month and torching it because of an hour-long commute. Some kind of assurance that things at XYZ company will be better than things at my current company, or that XYZ will be better for my career will help.
For me, specifically:
- 20% salary bump (one of the reasons I would leave is that I think I'm underpaid for my qualifications and market)
- small company (would like 12-16. I might consider a bigger company)
- no long daily commute (a couple days a week with telecommuting is good)
- talented co-workers
- product oriented company
There are other more subtle things, but these are the things off the top of my head.