With the girls, I had about 2-3 weeks of BAD morning (all day) sickness, with vomitting, etc. With this one I had 3 weeks of feeling terrible, but not throwing up. Don't know if this one's pink or blue yet.
I've heard that girls sometimes cause more morning sickness because of the slightly higher estrogen, or something like that.
Some people also go by the heart beat - theory being that girls are generally above 140 at rest, and boys are generally below 140 at rest (or thereabouts.) If that's true, this one's a boy.