Here's an idea that also works rather well. Take 
a thin sheet of paper, or some dental floss, or 
anythign equally as thin that you can slide 
between the sketch and the cel. Then, slide it 
between the sketch and the cel while gnetly 
pulling upwards on the sketch (cel is face down). 
This should work pretty well...always has for me 
^^ But whatever you do, DO NOT DUNK IT IN WATER 
O.O The paint (and ink) on cels is not very 
resistant, since it was made to be a temporary 
image (it only had to last until it was filmed). 
You will no doubt do damage to it if you dunk it 
in water as the paint might chip/come off. Also, 
when acetate dries after being very wet, it 
warps. I don't know about steaming, but it sounds 
like it has the potential of hurting the paint of 
or mildly warping the cel as well. Just use your 
discretion ^^

-Misha