Nail Clipping
This is not for the squeamish, or faint-hearted! Cutting rat's nails can be a tricky business as they are very wriggly, and if you cut too far down you can cause bleeding (not a good idea, as rats can lose a lot of blood in a very short time!). If you are unsure or anxious about doing it yourself, always go to the Vet.
The best implement for cutting toenails, would be a human nail clipper (not too big a one, so you can see what you are doing around it). Get someone else to help you to hold the rat still if you are not adept at this.
Splay the rats foot out so that the nails get extended. Take the nail clippers and cut the top sharp edge off. You may have to make a few attempts if the rat squirms a lot.
You should be able to see the quick of the nail pretty easily (it should have a pinker colour than the rest of the nail). If you can't see this, make sure to cut as little of the nail as possible. It is always better to cut little and often than to cause your rat to be sore and/or bleed.
You could also use a emery board, but if you are like me I don't like the noise! To try and prevent the nails getting to long in the first place, try placing a brick or other rough edged (not too rough!) object in the cage for them to climb over. I wouldn't recommend using sandpaper as it will get peed on and may hurt the rats feet as it can be really rough.