I've had my ACE250 for about 2 months now, and allow me to throw in my two cents:
1. You MAY get frustrated at first, but once you learn to use and master pinpointing, your finds will get better and better.
2. I agree with what was stated above - keep your sensitivity at about 4 (under normal hunting conditions), and hunt in coin mode.
OR
3. When you are first starting out, leave it in all-metal mode, in order for you to learn to dig as much as possible. This will allow you to master the different sounds it makes, so you know what to listen for.
4. Watch the DVD that your ACE 250 came with over and over, and bench test many different items, again, to learn what the different sounds they make.
5. Know that your MOST LIKELY best finds will come when you least expect it, and know that the older the site is, the more likely that there are older coins there.
6. Always remember - ASK PERMSISSION before you trespass, leave the site better than you found it, always be courteous, and dig as little plug as possible. You will find that the better you get at it, the smaller and less noticaeable your plugs and holes will become.
7. Research! Reaseach! Research! Look into where that old hotel, swimming hole, gathering place, etc etc once was "back in yesteryear", and search those places. You'll most likely find the good stuff, and learn a whole lot of great local history in the process.
8. Ask questions, and seek everyones advice and opinions on here. As you have seen, there is a wealth of great people, knowledge, and users on here that will help you with all aspects of MD'ing.
9. When in dount - DIG THAT SIGNAL! the one signal that you DON'T dig may be your next (or first?) huge find!
10. HAVE FUN!!
11. Join your local Historical Society, or Metal detecting club in your town or city. Again, you will learn alot, and you will discover many potential digging sites along the way.
Best of luck, and HAPPY HUNTING!!!!