1. Play the part of being their customer, tell them that they suck and that you know how to do it right.
2. Remind them you are one of the biggest consultancy firms in the world, showcase what you have done for other customers in the same industry and then convince them to buy the same solution.
3. Be humble to their challenges, recognize that they know their business better than you do, point out a possible direction where they can go and then convince them that you can help them go there.
Each one of these approaches might very well work depending on the specific customer and situation. To mix and match, picking the best from each approach, could also be a recipe for success. But deep down inside I believe in being humble.