Be generous. With your time, your resources, your words and actions. I encourage you to go the extra mile. In Napoleon Hill's book The Law of Success he writes 'Render more services than that for which you are paid and you will soon be paid more than you render.' Or said another way 'Your true worth is determined by how much more you give in value than you take in payment'. 'Exceed people's expectations and they will pay you even more'. Taken from The Go-Giver.
At first it doesn't make sense to give more than you have agreed to but in the long run it will pay off. Just don't be doing it for the pay-off, do it for the fact that you want to genuinely add value to your clients. It might be as simple as offering advice or knowing another company who does something you don't. Over deliver on what you have promised perhaps.