How to fix my prom dress?

I have this beautiful prom dress but the back is a criss cross but i need it lower and my mom and i were thinking of having it get a hook & eye kinda thing so that the back straps can cross lower but they need to be tighter too because my fall out of the sides because its looser up top because of the way i am built.. anyone have any suggestions to help me fix this before prom ?!