I guess the main hook is that if you already learned and helped improved their business they'd be fools to let you go when they would have to hire someone else for about the same price or more, teach them the business all over again, and go thru all the trials and errors they had to go through with the analysis in the first place.
Well, that might not be technically true. We usually tend to think like that. And, some times, we go ahead and gamble and we might win most of the times. However, a client might think that if they got a good consultant once, they might get another one thinking that that new person can start from where you left at the same rate so they'll be ok for another couple of years. I saw that as well. I however saw some clients coming back after 3 months telling that they lost money in that approach and they would like to get the original people back. because they now recognize the quality of consulting they got.
