Following to the opening lead is a critical part of defensive play. Some people like to play count, some attitude, whilst the more adventurous like a combination of both. The method described in this article is becoming increasingly popular in this country - we give some of the basic principles. This article is aimed more at those already familiar with giving count or attitude.
The basic idea is that if partner leads an ace or a queen, you follow with attitude, whilst on a king or jack lead you will give count. Partner still leads top of sequences, or chooses what to lead with the ace and king (always the king from AK doubleton). Thereafter you fairly blindly follow the rule just given.
For example, if partner leads the ace and dummy goes down with Qxx, you would play a low card from three or more low cards, or a high card from a doubleton, trying to get a ruff. However, if KQx went down in dummy then you would revert to count, or give a suit preference signal. Other than this fairly unusual situation, the normal exceptions to the rule are when partner and dummy are known to have at least nine cards between them (eg Qxxxxx in dummy, when you'd assume partner has AKx, or Qxxxx in dummy after partner has bid the suit). Now you should revert to standard count. Also, if a singleton goes down in dummy in a suit contract, you should probably give suit preference instead.
Whilst it is clear that playing attitude on an ace lead is not always desirable, it is quite playable to stick rigidly to count on a king lead, and that is what I do with some of my partners. However, with others I have some exceptions here too. On a king lead against a suit contract then I play attitude if dummy has 3 or 4 cards all smaller than the ten, or headed by the ace, with the rest smaller than the ten. (As you only need to know about the jack when declarer has not got the ten.)
As part of this method, against no trumps, the queen should be led from weak KQ holdings (eg KQ9, or KQ10(x) if it is a short suit lead). The king is led from strong holdings (eg KQ10xx or AKJ10x), and asks for count and unblock - partner gives count or else unblocks the queen or jack if he has it (but not the ten). (As a further point, with KQJ9x you start with the king, and then the jack to ask partner to unblock the ten, whilst continuing with the queen would ask partner to keep his ten.) If you don't want partner to unblock, you lead the queen to ask for attitude.
As a final point - how do you decide whether to lead the ace or the king? Whenever you have AKQ of the suit you would lead the king, to ascertain the count in the suit, as you don't care about partner's attitude to the suit. With AK to six you probably want a count signal also. With fewer cards in the suit you are more likely to be leading the ace, though if partner has raised the suit, then probably the king would be better.