The key to developing melds is to look for and hold combinations that give you the greatest number of possibilities to draw the additional tiles needed. A combination is any two tiles which, with the addition of a third tile, will form a meld.
For example, if you have a black 1 and a black 2, you can form a meld if you pick the black 3. This is an example of the least possible combination to meld, since only one tile will complete it. Any combination of two tiles, such as the red 7 and 9, which must be filled with a tile that comes between them, is also a combination that you are least likely to fulfill. When you are trying to make any combination, you need to be able to determine the maximum possible draws that it may take to complete them. This is where knowing the odds will definitely come into play.
As your hand progresses, of course the odds will improve that you will make your combinations. Some of the available tiles will become dead in the discard piles as well. Perhaps others appear on the table in the open melds and may or may not be available to you, depending on the tiles in the rest of your rack. What will make you a better player then the people you are playing against is your ability to keep track of all the tiles that have been played and form your own combinations to keep the maximum number of useable picks available. You also need to be able to drop a combination when you realize that your odds are not with you. A sign of a good player is one that can change course throughout the hand in order to improve your odds of going out.
In most cases, you will start out with several combinations. AS the play continues you will have more opportunities to form additional combinations and melds. This is when choosing which combination you wish to hold becomes important. If your entire rack is made up of combinations and melds, and the draw gives you a choice of changing one combination for another, it is almost always best to make the change for the combination with the greatest number of melding possibilities. However, before you do this, you must make sure the change also has defensive qualities as well.
If by changing the combination you actually improve your opponent’s chance to make a meld then it would not be advantageous for you to change your combination. You may have fewer odds with what you have in your rack already, but it would still be better than placing a tile for discard that your opponent immediately picks up to use and go out. Therefore, the best plays for your combinations should be both defensive and offensive at the same time.