sign language--boon or bane?

i've been trying out sign language with maeve since she was 6 months old and only of late has she been able to sign back...which is supposed to be good! now she can tell me what she wants and neither will get frustrated. right? wrong! now that she CAN tell me what she wants, she does it ALL THE TIME, even if i don't really want to give her what she wants. in this case, milk. she signs for milk every hour and i'm practically dried up already! and in case i didn't understand her sign, Maeve punctuates it with "malk malk" and looks purposefully at me. i'm now trying to pretend i don't understand her sign, which defeats the whole purpose of me trying to teach her to sign to begin with. now to teach her delayed gratification.