I always offer the chance to buy 10 to 20 sessions at their current rate with the stipulation that the advance purchase is only good for the amount of time that they would have used them up under normal conditions. Any unused prepaid sessions are credited to their account after the time limit and they purchase at the new rate from then on.
But, my really long time clients are always one rate hike behind all new clients. So if my new rate was $75 and the last pre-rate increase was $72, the long term clients go up to the $72. And they stay there until the next hike when they would go up to $75.