Teeth and pliers tend to leave unsightly marks on the bricks.
Fingernails don't work too well.
What tool do you recommend to separate them?
You could try the great brick separator. Designed for this specific purpose, it's probably your best bet.
Or you could use, guess what, a new brick separator:
I just noticed this one appearing on BrickLink and have no idea in what set it will come out, or if it will be sold separately, or if it will work with jumpers.
I normally use two big bricks: one on the top and one at the bottom. Then you push down while rotating.
In addition to the Brick Separator, there are some parts that work well for this
There's the great LEGO Brick Separator that can be bought in the LEGO Online Shop:
i love this thing, it's just great if you want to take care of your nails and teeth.
I use a fingernail most times, or a guitar pick if the bricks are stuck too firmly.
I (and my sons) use a plastic knife designed for children. We found ours in IKEA (larger bricks for houses)
I have also used this lego tool with moderate success
A flat-style staple remover (as opposed to the jaws style) does well to maintain the surface of a stuck brickage. It also can be jimmied back and forth without much slippage.
You can always try the Official LEGO Brick Separator. It would probably take a couple to work best on 1x2s though - one on the top, one on the bottom.
Disclaimer: I haven't tried this personally.
You could use two removal-tools (one to grab the top piece, one to grab the bottom piece)
I've separated several plates (and bricks too) with a simple spring-based hair clip.