Take a step back and look at your sprite up to this point. Determine if anything looks off, and then go into correct those inconsistencies or errors. After you've finished coloring and shading, you can add details such as writing, eyes, additional features, and anything else that could take your sprite to the next level. The attention to detail at the end is what separates amateur from professional pixel artists. If you've followed the steps above, you'll have a single static sprite. This is fine as a piece of art, but if you want to make sprites for games they'll likely need to be animated. This means that each frame of animation will have to have its own sprite, with slight changes from the previous frame. The collection of all the sprites used in an animation is called a "sprite sheet".  See this guide for detail on animating a sprite sheet in GIMP. Creating unique and attractive animations for sprites is one way that master pixel artists separate themselves from amateurs. A good animation can breathe a ton of life into a sprite. If you want to take your pixel art skills into the video game development world, you'll want to have a solid portfolio to show off to prospective employers. Include several of your best sprites, as well as some animations if you have them. Include a variety of subjects, including characters, scenery, props, and more.

Summary:
Give the sprite a good look over. Add details. Animate your sprite. Create a portfolio.