Since JJSB suggested I might comment, I will. I agree with most of The_Hog's answers.
First what I like to play most (I play all)
UDCA
2nd/4th best leads (!) (what another one??)
attitude, count, suit preference signals
Some special honor sequence leads
trump echos
And with some partners upside down smith echo
Second, for my opinions of UDCA, I think it does have a slight technical advantage over standard carding. Maybe we can start a thread just on comparing the merits of one particular optiion (like UDCA versus "standard" versus maybe O/E also known as roman).
Third, I already covered my views on 2nd/4th best leads in a thread started by EASY in the Advanced/Expert forum entitled "Opening leads of Low from 2nd and 4th".
Fourth... when playing so many what does a particular card mean? Is it attitude, suit preference, or count? Most signals I give on a hand is count, but ususally the very first signal is attitude. What do I mean?
When the are playing a suit, I generally give count. When partner leads a suit, I generallly give attidute, discards or second spot card in a suit are generally suit preference signals. None of these are absolute. That is, I try to give the signal partner needs to see. If I think a continuation of the suit partner just lead is never going to right, and FROM Dummy and bidding, partner knows this, then I GIVE suit preference, not attitdue.
If partner KNOWS how many cards I have is a suit, for sure, from the way I bid, then the card I play if spot is generally suit preference. This is fairly rare when playing standard, but playing Mulderber for instance, if I open 2S or 2H, I have EXACTLY 5 cards... so giving count is just silly.
If you give frequent suit preference signals, then sometimes you simply have to play a card and you have no suit preference. If your partner is going to believe every card you play convey;s suit preference signals, then you will get some very bad shifts from him from holdings he should not lead from. So try to find a sequence of plays that either cancels the alleged preference or play a card that give a suit preference that your partner can obviously see can not be a real suit preference.
Giving count. Sometimes you try to win the first lead of a suit (or in fact play high and do win). Obviously, you gave no attitude, suit preference or count signal. In this case, the second time I play to this suit, I generally try to give count. The way I do this is with REMAINING count signal. That is if I had an even number of cards originally, when I give count on the next round of the suit, I give remaing count (showing odd number of cards remaining) rather than ORIGINAL count showing even number to start with. This is not standard I thiink, but makes more sense to me. This has to do with returns, for instance... if I had A432 and partner leads the suit, and I win ACE.... if I return the suit, I return the 3 from 432 because I play 2nd/4th leads. so 2nd from 432 is 3. If I held Q82 and partner leads the suit... and I play the QUEEN to force the KING. On the next round, I play the 2 showing an even number of cards remining (I play UDCA) rather than the 8 (to show originally 3 cards).
LASTLY, sometimes it is perfectly acceptable, even necessary, to lie to your partner with signals. Sometimes because a little white lie will surely get a specific defense you can see that will be the winning line. Sometimes because you know from the bidding and your hand, that your partner will not be participating in the defense, so giving a signal only can help declearer. And sometimes, a false count against a declearer who watches such things, is the only way to try to presuade him from a winning line. False signals, sadly, do no good against novices and most beginners, as they don't realize you are trying to give a signal in the first place... but intermediate who has learned signalling well, you can usually lead them astray with such a ploy.
Now, what should beginners play? I think this...
1) Attitude on partners lead - unless very obvious switch needed
2) Count on declearers lead
3) Lavinthal or O/E discards
4) Trump echo only with ruff ready to go
If you like, you might make one exception to 1... on lead of KING by partner you could decide to always give count. Then partner, with AK wanting count (say against slam) could start King, and if wanting attitude, could start ACE, as an example.
Good luck.. figuring out what signals you want to use regularly and experimenting with a few others, will give you a lot of new enjoyment to the game while on defense that you may have been missing until you start using such carding.
Ben