If your new logo includes a symbol that can be used alone, be sure to get separate files (.eps, .jpg & .png) of the symbol.
Always ask for various formats of your logo when hiring a designer. A vector file is most important (.eps), but ask for .jpg and .png files as well.
Ask for your logo in various versions when hiring a designer. A good, reputable graphic designer will supply your logo as full color, black & white and reversed in all formats (.eps, .jpg, .png as a minimum).
Always ask for a vector version of your logo when hiring a designer. If a designer only gives you an image that you use in your Word or PowerPoint slides or on your website, what happens if you even need to make a modification to the logo down the road and can no longer contact that designer? You will end up paying another designer to recreate the logo, essentially paying twice for the same work. A vector file will have an extension like .eps, .ai, or .pdf.