A lot of people keep making the same blunders over and over when it comes to spriting - or working in MS-Paint in general.
- Unless you can re-draw and every line until you have a perfect piece of black-and-white artwork (no gray pixels floating around whatsoever), do not attempt to use MS-Paint to colored scanned artwork.
- Unless you REALLY need them, do not use the colors on the default palette. Alternatively, double-click a pre-existing color, click "Define Custom Colors," and choose your own.
- Avoid using colors with a maxed Saturation value. (Almost every default MS-Paint color has this.)