Trade

Never give an inch: trade it.

For example: when bargaining, don't use language like "I'd be happy with £X per day" when instead you might say "if you give me £X then I'll give you Y."

Appreciation

The question is - do I appreciate why I am being consulted? If so, I can charge the client a percentage of the gross value of my advice in fees. If not, I'm either going to be taken for a ride or I'll settle for low-fee work - either way I waste my valuable time. The Economist published this quote that sums up my feelings nicely:

A consultant's expertise is only valuable because it saves the time and expense of acquiring it ourselves.