I indicated to my boss last October that I wanted to leave and gave several reasons. I informed him I was looking for another role however it was amicable and I really liked working for him and thats why I wanted him not to factor me in to longer term projects as that would not be fair and it also gave them more time to consider my replacement.
I handed in my notice 8 weeks later and worked my full notice.

When I moved to Oz I kept quiet until the moment I had flights booked and visa in hand despite knowing for 5 months I was going. Timing was down to visa office and there was a chance my boss could have flipped out/made life crap. No point until I knew it was all sorted before telling them.