When counting, it is easiest to count with smaller numbers. When counting trumps, the simplest method is to count how many trumps the opponents hold. Then count rounds. If the opponents start with 5 trumps and both follow to two rounds of trumps, there is only one outstanding trump in their hands.
The same procedures can (and should) be used for other suits. Counting rounds is much simpler and much easier to remember than adding up 4 + 4 + 3 + 2 (or whatever) to get to 13. Make a mental note of how many cards you have in each suit—and how many cards dummy has. (Remembering patterns can help your recall. Perhaps your hand is 5-3-3-2 and Dummy is 3-4-4-2. Thus you can work out how many cards the opponents have in each suit. Suit patterns consist of three evens and one odd; or three odds and one even.)
If an opponent has preempted or shown up with a long suit, it is usually easiest to count that opponent’s hand. (Once you have counted one opponent’s hand, you know the hand of the other opponent by elimination.)
Start with any information you have from the bidding. One opponent has often shown a 5-card suit by opening a major, or at least 5-4 by opening a major and showing a second suit. Many bids, such as Michaels and Unusual 2NT show 5-5 hands. Preempts describe hands with specific length in one suit. Put that information in your memory banks to use when counting out the hand. Learning hand patterns will help you. Of the 39 patterns, one of just ten (4-4-3-2, 5-3-3-2, 5-4-3-1, 5-4-2-2, 4-3-3-3, 6-3-2-2, 6-4-2-1, 6-3-3-1, 5-5-2-1, 4-4-4-1) will occur 91% of the time. Once you have count of three suits in a defender’s hand, a mental pattern look-up for the fourth is faster and less taxing than summing the three suit lengths and subtracting the total from 13.
When you have a two-way finesse (e.g., KJ109 opposite A543), it is vital to put off the guess in that suit as long as possible. Forcing an opponent to lead the suit via an end play is ideal, but if you cannot manage an end play, count the other suits first. Once you know how many cards the opponents have in the two-way finesse suit, you may know that one particular opponent is favored to hold the missing honor. (If one opponent has 4 cards in the suit and the other has one card, you have a 100% play. If one opponent has 3 cards and the other has two cards, the one with 3 cards is a 3 to 2 favorite to hold the missing honor.)
Play the suits that DO NOT MATTER (where no guesses are involved) before you play the suits that do matter (where guesses are involved). If, for example, you will make your contract if either of two suits divides 3-3 and one suit has AK32 opposite Q65 while the second suit has AK102 opposite Q65, play the first suit first. There is no guess involved. Either you get a 3-3 break or you do not. Play the second suit LAST. If you can get a count of the hand, you may know whether a 3-3 break is possible or whether the opponent in front of the AK102 has four cards in the suit. In the latter case, you will finesse for the Jack if it does not fall earlier.
Incidentally, as a defender, you need to be thinking about when Declarer will be counting certain suits and refrain from giving accurate count. If your count information will be helpful to Declarer and is not essential for partner to know, lie! (But make a pattern of inconsistency so Declarer cannot be sure when you are signaling accurately and when you are not.)
Counting the distribution of suits is difficult in the beginning, but becomes easier the more you practice. It will pay great dividends for your Declarer play and defense to count suits.