I have a Smith's Knife and Scissor sharpener. Here's a pic:
http://kennesawcutlery.com/Sharpeners/Smith's/Smiths-Knife-and-Scissor-Sharpener
You can get them at Lowe's, Home Depot etc. They really work well. Just a few strokes and you can really tell the difference. I run my scissors thru every time I put them away so they are ready to go the next time I use them. The knife sharpener part is also terrific. I also sharpen my knives after I wash them, just before I put them away. This tool actually just dresses the cutting edge. If your scissors are really dull (and they are good quality) I would have them professionally sharpened, then keep them dressed with the tool. Won't need professional sharpening again.