Not bad at all for Paint. Other than practice, and what Yoru Ryu said, try to adjust the line widths, using thinner lines a bit more than thicker ones (Detailing, etc.)
I hate Paint, it seems so uselsss, so I'm suprised you were able to do all of these. Good work, next time maybe use shading, different thickness of lines (broken record, I know, but what else can I say?)