My next guess would be that you don't know about the advanced curve feature. There are two types of curve to the line, one which will strictly follow the seeds (this is the one everyone usually knows about), then the other, where you left click an end seed, hold left click then right click and release on the seed, then when you curve the line by dragging a seed, the curve is not as strict and the rest of the line follows. Knowing how to use both features makes it very easy to draw things like hands or eyes without having to use as many lines or freehand.
I don't think your problem is anti-aliasing, I think its just because you are using freehand on some parts, where you could have used the line tool in ways like I mentioned above. It's hard at first, but once you get used to it, it becomes easy to make very clean line-work. Freehand should only be used if you have a tablet IMO, or for a rough undersketch which will not be shown in the final draft. It also helps to go back over the ends of thick lines, and make them thin, by using a white line to erase it (if you layer properly, this white will not appear on anything else).
