By popular demand, here is a rundown of some simple defensive signals.
When partner leads a suit, to encourage the continuation of that suit (if, say, you have an honor in the suit, or a doubleton with a chance to trump), discard high and then low.
To discourage the continuation of that suit, discard low and then high.
When declarer (or dummy) leads a suit, or when attitude is obvious or irrelevant, we signal to show count, not attitude. This helps partner figure out how many cards in the suit declarer holds. Discard high and then low to show that you were dealt an even number of cards in that suit; discard low and then high to show an odd number.
When leading a card to partner for her to trump, lead a low card to request the return of the lower of the two other side suits; lead a high card to request the return of the higher one.
For example: if spades is trump, and you know partner's out of diamonds, lead a low diamond to indicate that you have the ace of clubs (clubs being the lower outstanding suit). Partner will trump the diamond and lead a club to you, which you win, giving you a chance to lead another diamond for another trick. If instead of the club ace you have the spade ace, you should lead a high diamond instead of a low one.
Lavinthal is a special suit-preference signaling convention. When you can't follow suit, you can indicate which other suit you prefer. It's used only the first time you can't follow suit and must make a discard (often when declarer is pulling trump). Discard a card from a suit you do not like. A high card indicates interest in the higher of the other two suits (not trump and not the one you're discarding); a low card shows interest in the lower one. For example, if spades is led and you are out of spades, discard a low club to show interest in diamonds.
After your first discard, a high card in a suit signals that you would like that suit led to you; a low card in a suit signals that you do not like that suit. So for instance if spades is led and you are out of spades and like diamonds, but can't part with a high diamond to show that preference, discard a low heart (and perhaps next a low club) to show your distaste for those suits.