Jiu Jitsu Half Guard

I know that one aspect of Jiu Jitsu that you favor a lot is your half guard. It's a great position for everyone, whether you're a beginner or a higher ranked guy. What do you feel are some of the technical fine points to develop an effective half guard? I think the main thing is you can't let people grab your head and you can’t let people take underhooks. And then you can be on your back as long as you don't let those things happen.