This topic separates the amateurs from the experts.
There is no shortcut.
It takes lots of concentration and experience.
This article is intended only as a very brief introduction/summary -- and the chance to play 4 practice deals at the end.
As declarer, here are the main things to focus on:
1) In a trump contract, always count the trump suit. The best method is not the "fingers method." In other words, don't count "1-2-3-4" for the trick you see in front of you and then add "5-6-7-8-9" adding the 5 trumps in your hand. Instead, just take note of your side's trump fit. Say you have 5 and dummy has 3. You have 8 trumps. Think in terms of "How many trumps are missing and how will they split?" In this example, 5 are missing, and you usually hope they will split 3-2, but sometimes they will be 4-1 (and rarely 5-0). As you play the suit, just watch to see how the 5 trumps fall out of the opponents' hands.
2) It is hard work to count every suit. With time and experience you will recognize when dummy comes down which suits you need to count. In a trump contract, of course trumps is one of the suits you'll be trying to count.
3) Use the bidding to help with the count. If West opened 2, it is likely you can place him with 6 of them. Subtract from 13 the number of hearts you see in your hand and dummy. Then you know how many hearts East has.
4) The best way to get a count is when an opponent shows out in a suit.
5) The purpose of counting is that "late in the day" you can make the right play. Try to play the "easy" suits first to get a count on the "difficult" suits.