Only 4 month tenant wants to move out

[Liz C.]

Hi all, My tenant only been place for 4 month want to move out and is giving me 30 days notice. The lease say she can move out by giving me 30 days notice but I can keep me 1 and half month deposit . And now she is ask me if I am kind enough to return her deposit if I can find next tenant before she moves out. Is just a lot effort for me to find another tenant and I can't believe she is asking me for deposit back. Should I just star looking for next tenant ? And should I return her deposit if I find replacement? Next time I will change lease to if tenant move out before the lease they have to pay the rest of year ! What should I do to prove that such situation from happening again?