Start with a soil test.  There is a reason that the alfalfa is thin.  Make sure it is not a fertility issue.

If it needs lime, you should be getting lime on 6-12 months ahead of seeding alfalfa. 

The other nutrient I would pay attention to is potassium. Alfalfa is a huge consumer of potassium. It would not surprise me if the soil potassium level is depleted.  If so, get it build up before you reseed.