I do not counsel law departments to freeze billing rates of their law firms across the board (See my posts of Nov. 11, 2007 about broad-brush measures; and April 26, 2006 on billing-rate freezes imposed that way.). I do, however, feel that it is fair to ask a law firm that you have selected to handle a major matter to freeze the rates of the core lawyers who are working on that matter for its duration. Or perhaps for 18 months.
You build in a back-end discount then. The arrangement also gives the law firm a slight incentive to resolve the matter more promptly, before their normal rate increase takes effect and they lose that bump up. The firm might try to limit the size of its core team just a bit (See my post of Dec. 8, 2006 about core teams in law firms.), since its members will see their charge-out rate stay flat on that matter.