Tactic #1) Negotiate on 'value added'
eg talking to the client you might say...
"Last year you received £x of profit as result of working with us and the fee was £y.
"This year is the year of added value and we have found ways to generate £x + 20% of profit for you and the new fee will only be £y + 10%... so you are getting even more profit for your investment."
This is not as daft as it sounds but you have to ask for the price rise (and justify it) to get it.