I spent about half of my career working for Kansas City's Metro Information Services, then Keane (when they acquired MIS) as a consultant. As a consultant, I eventually learned a crucial truth which applies to consultants and employees alike. It's just that employees have the luxury of forgetting this from time to time.
The Consultant's Law
As directly as possible, ensure that your efforts make money for your clients.
That's the secret to really long contracts and an high level of control over your career. There's a well known evil doppelganger of this law that I'll call The Law of Job Security.
The Law of Job Security
As directly as possible, ensure that your absence will cost your clients money.
A lot of people seem to live by this law. It's subtly different from The Consultant's Law, but The Law of Job Security breeds resentment and comes across as unprofessional and opportunistic. It's also dangerous, because as irrational as it is in some cases, people tend to punish those who take advantage of them - even when it costs them and their company.