What factors go into making a decision to move your company to another state?

Labor market is key input from an HR perspective both supply and cost of labor. Other secondary factors are labor laws in the state as well as any tax breaks the localities may offer for business willing to relocate to their area. ERI Economic Research Institutes provide provides geographic cost of labor differentials and many clients use this to manage multiple operating units' compensation costs. Hope this helps.

Are you moving the company, or just the legal entity?

Malak points out some great items, especially from the tax/HR perspective.
I would add to his list:
-Access to resources (bandwidth, water, inputs, whatever it is you consume)
-Access to your customers
-Access to investors in that space (especially for startups)
-Presence of competitors (which means a lively place to be in your type of business)

If you are just moving the legal entity...then it is "whatever state supports your type of entity". The answer might be DE, CA, VA, FL, NV....your tax attorney may surprise you with how differently each state will treat different types of entities/products. Offshore is also a great answer if you have an international presence (keeps everything very clean).

In cases where I have had to do this: labor supply (especially availability of highly trained, technical staff, proximate to training universities), tax jurisdiction and benefits, local government relo inducements, environment for litigation and tort reform were guiding influences.