As has been mentioned, you can spend a lot of money on an implement to help level your arena. The Reveal 4-N-1 is a great implement to smooth, level, and groom your arena, but it is also fairly expensive. A heavy chain drag/harrow can help even out an arena, but you really need to drag as often as possible. There are also some straight blades with a few modifications on them that are used in arenas.
Your best bet (if you wanna stay cheap as possible) is to use a box blade or straight blade type arena tool to smooth out the arena and get it level. Afterwards, keep after the footing by frequently dragging with a chain harrow/drag. By doing this, you will probably only need to box blade/straight blade once or twice a year while keeping up with day to day operations via a drag.
If your arena is sand, you'll want to turn it over every once is awhile. I usually use a disc. As the horses work over the arena, they grind the sand particles into ever smaller sizes. This is very evident when you see a 'haze' of floating dust while riding. By turning over the sand, you send the smaller particles to the bottom and bring the bigger ones up to the top. It also helps the footing, but it might take some dragging to get the depth right.
I don't know what kind of condition your arena is in, so I'm not sure exactly what you need. Some people can get very reasonable results by attaching their drag to a railroad crosstie. Others aren't so lucky... /forums/images/graemlins/wink.gif
Hope it all works out for you.