Lightroom color space settings
One of the simplest and yet most impactful things you can do to set yourself up for good output files is to make sure that you're editing your files in the sRGB color space. All web browsers render files in sRGB, and the vast majority of print labs print in this space as well. Saving your files with the wrong color space embedded can result in gross gray-green desaturated colors, so be sure to embed sRGB at all times!
Here's an example of the kinds of grossness that arises when you've got the wrong color profile embedded in your files: