How can I keep corned beef pink?
As Jay mentioned, potassium nitrate or sodium nitrite is what gives commercially made corned beef its long-lasting pink color.  Home cooks can use the same chemicals.  Just make sure that whichever you buy is specifically labeled for use in food.  Also, since you mentioned that the corned beef you buy comes with a spice mixture, check the ingredient list to make sure it doesn't already contain either chemical.

I've only ever used potassium nitrate (A.K.A. saltpeter) for corned beef.  1-2 teaspoons per gallon of water is plenty.