You can take a tortillon and go over anything sketched to smooth the shading and lines. HOWEVER... 1. they cost money, however little. 2. they can be tricky to find, depending on what stores you have nearby. and 3. it's cheaper to just take a scrap of paper, and start folding it like you would make a paper airplane, and keep folding it over and over until you get a very narrow point/concentrated bit of paper. What you really need is a point that has some manner of substance to it, so you can apply a bit of pressure.
It can take a little getting used to, but if you work on the principle of "less = more" then you'll do just fine with it. Start off with keeping the most graphite/lead/charcoal nearer to the outer/darker areas, and begin blending your way in. You can smudge the color over a pretty sizeable area, so work on applying the most color almost as an outline, and then smudging and working it inwards. Don't make any harsh/sharp lines, and use a very light hand to put a basic layer of color down. Sometimes, I literally only use 3-4 pencil strokes over a centimeter or so of paper, and then buff over it with paper to disperse it. Also, after you've been using it a bit, your paper/tortillon will be "dirty" and will have the ability to transfer color without a layer of pigment already on the paper. This can be great for medium-darkness areas, but can mess up a light area quickly. That's another reason why I use scraps of paper, as I can change them as often as I like.
It's also possible to use an erasing technique to leave a smooth, faded color/shading. But that's a lot trickier to control the lines of, and after a while, will remove part of the paper. I don't recommend it unless you're positively desperate to remove color from something.
