By Rabbi Julian Sinclair, March 6, 2009
Tizkeh lmitzvot literally means May you merit [more] mitzvot, and is used as a form of thank you among religious Jews. If someone gives you a lift home, for example, you might thank him/her by saying tizkeh/ki lmitzvot.
As it is a type of blessing, the proper response is Amen. Some add a blessing in return, Tizkeh laasot, meaning, May you merit the performance [of mitzvot].