Here's what I tried, it really helped me to dig A LOT less trash-
Every time you get a signal, pay VERY close attention to it. The way it sounds, how "big" it is(how far you can move the coil and still get a tone), where it comes in on disc.(target id), how deep it is, then dig it. The only way to know for sure whats down there is to hold it in your hand, but if you learn how your MD re-acts to different targets, you will start to get an idea of what you are looking at while it's still in the ground. The only way to figure that out is to remember what the target sounded like BEFORE you dug it.
Here's a little story - My tesoro is very frequency specific, it reads the magnetic signature of a target, not necissarily(sp?) the metal type/size. Therefore, when I get "near" iron, it comes up as silver, after a little pinpointing, it usually reads a lot truer. So usually when I get a nickel signal that changes to iron/foil, it turns out to be just that. The last couple days I have been in an area that has been giving a lot of nickel signals, when I pinpoint them, they start to read zinc penny. They are showing up at 3-6", a little deep for clad, and corroded pennies can sometimes(on my machine, I know from past experience) show up as zinc, so I started digging these "almost junk" signals, and so far I have 6 wheats and seven Indian pennies to show for it.
The object isn't to dig TONS of coins EVERY time out, at least, when you're getting started. Worry more about learning how your machine "sees" things, when you learn that, you'll start digging A LOT better stuff.
P.S.- Shootist is usually a little "touched", but that advice about reading a target that has 4 different IDs, so far as I've seen, thats pretty much right-on.