First of all: If you're not signaling to your partner on defense—and using your partner's information in turn—you're not defending properly.
Declarer can see both of his side's hands. Defenders need to convey as much information as quickly as possible to partner, and that means efficient signaling is a must.
It amazes me how many pairs still play standard attitude (high card encourages and low card discourages) and standard count (high-low shows an even number of cards in the suit, low-high shows an odd number).
To understand why so many experts prefer “upside-down count and attitude,” consider the following:
If you have a good suit—say, KJT94—do you want to waste a valuable high card telling partner about it? Of course not. With upside-down attitude, your lowest card tells partner you like a suit, and you keep all those nice high cards.
Conversely, if you have a lousy suit like 863 you don't mind tossing away a “high” card.
Consistency is a good thing, right? You're probably already using upside-down attitude! When you lead a new suit to partner, it's standard to lead low if you like the suit and high if you want partner to shift. This is the same principle at work: You don't want to squander your strength.
With only two cards in a suit, which do you want to play to give count—the highest or lowest? Again, you don't want to waste a potentially valuable card, so playing the lowest makes sense. This is upside-down (low-high shows an even number).
With three cards, you can more easily afford to play high (sometimes the middle card is high enough.) So high-low makes more sense for an odd number of cards.
Leading from a Doubleton
Like many standard count/attitude players, we used to worry about what to lead from three small cards. Lead the top card, and partner expects a doubleton; lead the bottom card, and partner expects an honor. Play MUD, and partner is just confused.
One day we woke up and realized an old habit was blind-siding us: Even though we play upside-down count, we were still leading high from doubletons. Now we lead low from doubletons, and our problems are over. All low leads are encouraging, whether it's from shortness or strength, and all high leads are discouraging. It's consistent, and it's clear.
Note that leading low from a doubleton is the only carding agreement requiring a pre-Alert in ACBL-sanctioned events. Since we don't always lead low—we'll lead the higher card to unblock or discourage a return of the suit—our pre-Alert says “Low from xx asks for a ruff.”
4th Best / 3rd and Low
It took me a long time to get a decent explanation of why many experts prefer “3rd and low” (in which you play your 3rd-highest card with an even number of cards and the lowest card with an odd number of cards) over the traditional 4th best—but often only at suit contracts. What it boils down to is that 4th best clarifies the leader's strength at trick one, while 3rd and low clarifies the leader's shape at trick one.
At notrump, partner needs to know the location of missing honors to judge a hold up or discovery play. That's not such an issue at suit contracts—3rd hand will almost always play high. More important is to know when Declarer might be ruffing the suit. Here, 3rd and low does a better job of showing the opening leader's exact length (at the expense of locating missing honors).
We play 4th best at notrump, 3rd and low at suits. But unlike upside-down count and attitude, the advantage here is minor. The best advice is to use whatever you're most comfortable with.
A from AK
If you lead the K from both AK and KQ, how does partner know which you have? We lead the A from AK at suits, just like any other 2-card sequence.
0 or 2 Higher
Traditional leads from broken sequences like KJT or QT9 are to lead the top of the touching cards (the J from KJT, the T from QT9). As with K from AK, the problem is that partner doesn't know whether you have the higher honor or are leading top of a sequence. You might have a higher card, but it can be very difficult for partner to judge who has that one missing honor.
It's much easier for partner to notice whether you're missing two higher honors. So we play “0 or 2 higher,” which promises exactly what it says: When you lead a jack, ten, or nine, you have either 0 higher cards or 2 higher cards. We lead the T from KJT and the 9 from QT9. (Another name for 0 or 2 higher is “Jack denies,” because leading the J implies 0 higher honors—you wouldn't lead away from from AQJ.)
The only drawback is that some experts believe 0 or 2 higher gives away too much information to Declarer. Larry Cohen facetiously suggests playing it only against non-experts, when you will help partner more than Declarer. Since we don't get to play against experts that often, and since in general the defense needs to share as much information as possible, we like this agreement.
Having tried many different signals for the first slough, we've settled on “odd-even discard.” With this agreement, discarding an odd card says you like the suit—the lower the value, the stronger your preference. An even card says you don't like that suit, and the relative highness or lowness indicates whether you prefer the higher or lower of the outside suits.
We believe this is the most flexible signal, with Lavinthal a close second, because you can pick a signal from any suit.