Well, we had a little discussion on drawing with tablets here:
http://www.gaiaonline.com/guilds/viewtopic.php?t=17790659Other than that, I can't really think of any tips on using a tablet.
In the first picture, the face doesn't really match the head shape/angle... and I think the eyes, nose, and ears are too high (the nose more than anything). My thoughts on face proportions are better explained here:
http://cloudflash.net/arttips.phpThe body/clothes look good, though (though the whole picture is pretty sketchy).
The hands don't really make sense in the second picture. I can elaborate if you want, but it's basically as if the back of the hand is showing, but making a heart like that should reveal the palm of the hand.
Hard to tell what to make of the arms in the third picture, but it looks good overall.
Fourth picture seems to be intentionally lacking proportions and stuff... no comment.