Lines with tablet always look jerky?

I draw with a Wacom Bamboo tablet and a array of different programs (I've tried gimp, inkscape, illustrator, SAI) and no matter what I do, no matter what brush I use, the lines always look jerky. I've seen other people's work with the same materials and even simple quick sketchy lines always look smooth and clean. Mine literally come out looking like when you have to sign using those crappy signature pads for credit cards at stores DX what am I doing wrong?