There are many strategies for open melding, but the important aspect to know is that you need to play them defensively. This means paying melds that are the most difficult to rearrange or add on to. The safest meld is a group of three. There is only one other tile that can be added to such a set by another player which is better than the group of four that allows any one of the tiles to be removed and used in another group or run. A group of three requires that all tiles be rearranged and used, which is much more difficult.
When it comes to runs, if you have to play them, you should again choose a run of three rather than a run of four. An opponent can lay off at both ends of the run but cannot take an end tile to use in another set. A run of five buries one more tile than does a run of four, so that may be a choice as well. Again, two end tiles are available for use in other sets, but the fourth is relatively protected. A run of four tiles is safer than a group of four tiles. With the group, any one of the four tiles can be removed and used. With the run, only the two end tiles are available. Then of course you have a run that begins with a 1 or ends in a 13. They are very safe because they can only have tiles added to one end, or taken away from one end.
This pattern can be used in the opposite manner if you are playing the tiles on your rack. In this case, you want to maximize your chances of creating new melds or adding onto existing ones. Therefore, you would attempt to form melds from the lower part of the figure in preference to those in the top part. Here you need to make a choice between offensive and defensive play. The strictly defensive player plays his rack the same way as he plays the open melds, knowing that sooner or later he has to play the melds on the table.
With open melding, obviously you lay off as many of your tiles on the table, as well as form melds on your rack. If you have, or pick up tiles that can be laid off to the open melds, it is best that you keep them as long as possible in your rack. As the hand progresses the chances of someone else having the same tiles you do becomes less, and you are almost guaranteed layoffs when you do finally need them. You may also prevent your opponents from using your tiles in open melds and keep them guessing as to what you are holding. Again, it is a matter of your having the knowledge in which they lack.
If there are a number of open melds on the table, such as what can occur with four players, you have the possibility of acquiring several such tiles. These are great for when you are ready to go out. The effect of adding just one tile to an open meld is important to understand, because adding to it changes its ability to be used by both you and your opponent. Adding to any open melds is a poor defensive move and has no offensive value whatsoever.