Jeep Freak 81
The best way is to polish it with some metal polish until the scratches disappear. I would suggest using some Mothers Mag and Aluminum Polish. It is available at most retail stores. It will be in the Automotive department. Also Brasso works, but the shine doesn't stay very long before it tarnishes again.

Michael
Most brass plaques have a clear coat to prevent tarnishing. The clear coat probably has the scratch. It would be repaired the same as a car, but it is a very thin coat. You would have to clear coat the face and buff it out with a very fine sand paper and polishing compound. If it does not have a clear coat, use a metal polish to buff out the scratch. A fine sand paper may be required if it is a little deeper.

fizixx
You can try the polishes mentioned in the previous answers. I would probably not recommend you use sandpaper, unless is was much, much greater than about 3600 grit. Even then I wouldn't recommend it. It's too coarse. Use the polishes, and when you are done, go over it with toothpaste. Don't get the gel type. Get the standard, regular Crest, Gleem, or Pepsodent.....the paste. Rub this around and it should bring it to a better shine because it is a very, very find abrasive. You should probably get some acetone (nail polish remover may suffice, but with no color or dyes added), and rub the metal over really well. D this in a really well ventilated area tho, and make sure you are ready to do the next step, otherwise the brass will begin to tarnish right away after it's clean. I would contact a hardware store, or home improvement store, or better yet, a metal works place and ask about a protective outer coating that is clear. You see, metals oxidize. That is; the oxygen in the air attaches to the metal surface and this is where tarnish and rust come from. When your brass 'tarnishes', that is rust, for brass. On steel it is the familiar brown-orange, flaky type appearance. The clear coat is a barrier to the air, either preventing, or minimizing this taking place, helping the metal surface to remain nicer for longer. But you need a metal-person to recommend the right stuff for you. Here's a guide below.