When to share that you are looking

Here is a cast about keeping it confidential from your boss if you're looking at moving to another company. I recommend not telling your boss if you are looking outside the company. If you tell him before you have an offer, then it gives him a big opportunity to be frustrated with you. He may lower your work load and start to transition that load to other team members (limiting your experience and growth). And most importantly, it would leave you in a very sticky situation if you end up staying at the company for a while. Looking for another position can last months, and until you've got something you've got nothing (with respect to an offer that is). 

Now if you are looking for positions internal to the company that is a separate matter. Your manager may be very helpful in that case. 

There are several casts that you should listen to as you start your search. You can find them all on the Map of the Universe in the Transitions section. In addition to those, the Interview Series is extremely effective. I've used it myself and have been able to receive the benefits. 

Good luck on your search!

Mark

I spent 4 years looking internally and externally. My skip manager, who became my manager a year before I moved, was very aware of my search and encouraged it. I will be the first to tell you though that he and I had a different type of relationship as he always wanted what was best for both the business and the employee. I have never run into anyone else like him.
He helped to coach and mentor me to help me grow in the directions I needed to in order to land my next role.
I agree that if it is internal, test the waters because your manager may be able to help you. Occasionally you may run into those who will actively seek to prevent you from leaving because they don't want to loose you. In the end they will most likely push you out of the company.
If you are looking externally, keep it on the DL until you have an offer that you have accepted.
Mac