Agreed! Saying no to a new client who contacts you is not too difficult (although depending on circumstances a pile of bank notes being waved at you can influence the decision...) - but it is much harder, at least I find, to say no to existing clients for fear of damaging the relationship.
That's precisely what put us in the situation we are now. Its fine, you build great relationships and can integrate a great many things, but finding the time for other projects just disappears.