How do I tell my boss I'm leaving for a competitor?

I got an offer from another practice in town (not a direct competitor, but same city). Better pay, better hours, seems like a better fit. My current boss has been good to me, but I need to do what's best for my career. How do I have this conversation without burning bridges? Do I give 2 weeks? 30 days? Should I be honest about where I'm going?