Summarize the following:
Before you even think about scratching anywhere, you should make sure your nails themselves are clean. Scratching with dirty fingernails will only serve to spread bad dirt and bacteria around. Soaking your hands regularly in hot water for a couple of minutes at a time will help open up the pores, releasing any excess dirt from your skin. The less bacteria you're tracking around, the better. Particularly when you're scratching, it's very important to take a close lens to the dirt trapped underneath your fingernails. Most often, this muck is difficult to see, so it's important that you give it a good look and proper scrubbing over.  This is especially true if you like to keep your nails fashionably long. Make a point every day of cleaning the buildup muck from underneath your fingernails. It will alleviate a lot of future hassle. Even if you like to keep your nails long, it is best to keep them filed. Uneven nails can feel rough against your skin, and might even risk snapping, given pressure. Make a habit of filing them once every few days; not only will they feel smoother, they'll look quite a bit nicer as well.  Trimming your nails is also recommended. While some people prefer longer nails as part of their style, it can add undue complications when it comes to scratching. Longer nails carry more bacteria, and may even run the risk of breaking apart if too much pressure is applied. Don't bit your nails. While it may seem like a nice alternative compared to trimming them with clippers, your nails will look nicer if you go the extra mile.

Summary:
Keep your fingernails clean. Soak your hands in water. Clean any dirt from underneath your fingernails. File your nails.