One of the best things you can do as a consultant - or in any job for that matter - is to strive to be replaceable. For some, this is counterintuitive: How can you be replaceable if you are really good? Don't you want to ensure job security by being irreplaceable? These sentiments are not uncommon, and sometimes you even see someone striving to make themselves ever less replaceable by sheltering information from others, or writing undocumented or complex code, or in other ways. Its no good though, and here's why:

First, if you are not replaceable then you are not serving your customer or employer very well. What if you get hit by a bus? Or quit? Or get sick? Or go on leave? Then they are in tough shape. You left them in a bad spot by being irreplaceable. Serve your customers by being open and sharing information, documenting your activities well and training others to do what you are doing. Try and proactively transfer knowledge if you can, and if not, start sending out pings up the chain to suggest that at least one other person share your knowledge and skill set.

Second, if you are not replaceable then you will have a hard time moving, advancing, trying new things, and looking like a positive team player. The only way up (or to other interesting places) is to NOT be locked into the position that you are currently in. Customers and employers usually realize when someone is serving them well (e.g. by making themselves replaceable) and so doing this serves your career interests again by generating positive sentiment among your customers.

Becoming replaceable is a little scary, but its a good way to serve your customer and its a good way to advance your own career and move on to new and interesting things.