[Rusty]

Quédese con el cambio. (formal or respectful speech)
Quédate con el cambio. (familiar speech - can be considered offensive if the waiter has not already addressed you in familiar fashion)

There are a few alternatives, but the sayings above are commonly used. In northern Mexico, you may hear older folks say 'el vuelto' instead of 'el cambio'.

It's uncommon to wait for the change to be brought to your table by the waiter and then say something about keeping the change. Say this up front, when paying.
If you're leaving a tip, and the tip amount exceeds the amount of 'cambio', add enough to the payment to cover or almost cover the tip and then indicate that you don't expect any change.