1. Early on, evaluate the potential of your hand. If you have a lot of pairs, plan to try for an all-pairs hand or an all-Pung hand. Consider whether you want to throw away terminals or simples.
2. Determine as early as possible what yaku you think you can get. Know if you have Dora or not but don't count on it for a yaku (it isn't). Don't forget that Reach is a yaku too.
3. If your hand is not leaning towards a high-scoring hand, go for the more common hands. Most of the time you will be trying for Tanyao (all simples) or Reach or Pinfu (all chows), or a combination of those. The one-fan yaku are the most common hands. Most of the time you should assume that is what you will make. The tiles that come into the hand may give you better opportunities.
4. The tiles you will throw are perhaps even more important than the tiles you will take. You do not have any control over what tiles you will draw, but you do have control over what tiles you will throw. Most of the time you will throw winds and dragons and terminals early in the game (remember, most of the time you are trying for simples and chows).
5. Do not be too quick to jump on somebody's discard-take it only if you've already got a plan for what yaku you're shooting for. (See next two for specifics.)
6. Don't meld Chows unless you have a definite yaku plan.
7. Don't meld Pungs unless you decide Reach is out of your reach.
8. Every now and then, go for it. Shoot for the really really big yaku hands. Pure, Clean, All Honors, etc. You only need one "really really big" hand to win the entire game. Two "just really big" hands can win the entire game too.
9. Know when to give up on your plan-watch the discards and melds to see if it'll be impossible to get the tiles you want.
10. As the wall decreases, wariness and caution should increase. Late in the hand, do not discard anything that nobody else has discarded. Even tiles discarded early in the hand can be dangerous, late in the hand.
11. If you can't win, at least try to go tenpai-but not at the expense of letting somebody else win.
12. If your chances of winning are low (and/or if your score will be low) shift your strategy to defense. Try to figure out what other players need to win, and do not discard that, even if it means having Noten.
13. Know your wind and the prevailing wind. Hang onto those tiles until you think they can't be used or conflict with your shaping hand.
14. Discarding tiles identical to what the previous player discards is always good, if you can.
15. The 1-4-7 rule is a good defensive discarding strategy for the dangerous phases of the game. If the player to your right discards a four, and you can't throw an identical tile, it might be "not too unsafe" for you to throw a one or a seven. Remember these sequences: 1-4-7, 2-5-8, 3-6-9 (each is a sequence with two numbers between). If a player has discarded both a one and a seven, it's likely safe to throw him a four. If a player has discarded only a one, it might be unsafe to throw a four (the player might have a three and a five).
16. Be aware of your ranking among the other players at the table. If you are in third place, it's better to have the second-place player to have to pay you off than the fourth-place player.
17. Try to go out waiting for multiple tiles (not just one). Imagine that you have three chows and two pairs. One pair is Two Bams, and you draw a Three Bam from the wall. Which tile do you discard now, a Two or a Three? In this situation (depending on the other tiles in the hand or on the table), many experienced players will discard a Two, keeping a Two-Three combo (an incomplete chow waiting for a One or a Four to win). A two-way call is better than a one-way call. (And this example would also put you in line for pinfu (all chows), a popular and easy yaku.)
18. At times it is important to ignore these strategies. You might never make certain special yaku if you never take chances that go against the prevailing wisdom.
For more about Japanese Mah-Jongg: