Never be indispensable.
Now, when I first heard this, I thought it was silly. This was Silicon Valley in the year 2001, after all. The only people not getting laid off were the indispensable ones, right?! Wrong. A lot of people were losing their jobs, some not so good, some very good, and sometimes it was everybody (yes, I did ride a startup all the way down).
The trouble with being indispensable is that you can't leave. You can't be fired, sure. But you can't take vacations. And you can't get hit by a bus (or chuck it all and move to Antarctica). You also can't be promoted.
Be very good at what you do. Be important. Be useful. Be a hard worker.
And then help others do what you do. Help those around you be good at what they do, and know how to do the things you know how to do. Teach others the tricks you know, and the skills you have. Help them replace you, so you can move on to new and bigger things.
Don't be indispensable. Be a leader.