outdoor light post rusted out at the ground. How to I prevent it the next time?
Make sure concrete is above ground.  3-4" is usually good.  This will keep standing water out.
Paint metal before putting it in concrete.  A metal primer and a sealant coat goes a long way.
Slope concrete form away from pole on the top 1-2 inches.  If you do this right the concrete at pole will be about a 1/2" taller than concrete at edges.  Your pole is rusting quicker because of standing water.
Caulk concrete to post gap.