Do you know the "rules"?

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"By the Numbers" Rule of 2, 3 & 4:
Pre-empt such that you expect to loss two tricks with unfavorable vulnerability or three tricks with equal vulnerability and four tricks with favorable vulnerability.

Rule of 7:
When declaring at no-trump and determining how many tricks to hold up, subtract the number of cards that you and the dummy hold in the suit led from 7. Hold up that many times. Hence if spades are led and you have six spades in both hands, hold up one round and take the second spade trick. On the other hand, if you have five spades between you, hold up twice and take the third trick.

Rule of 8:
When considering a two-suited overcall after a 1 no-trump opening bid, if the number of losers you have (counting missing aces, kings and queens) subtracted from the number of cards in your two longest suits is no more than two and you have at least 6 high card points, (2 + 6 = 8 hence the rule name) then you may make a two-suited overcall with a five-four distribution. (See ACBL Bulletin)

Rule of 11:
To determine the number of cards in the partner of the opening leader's hand (or in the declarer's hand) of the suit in which the opening leader lead fourth down, subtract the number of the card led from 11. Reduce this count by the number of cards you see in your hand and the dummy that are higher than the card led, the result is the number of cards higher than the card led in the other opponent's hand. Note that you may find from this exercise that your partner did not lead fourth down. This is also a valuable insight.

Rule of 13 for opening strong 2C:
To be used when you have one of those distributional hands with less than 22 points, and you are not sure whether to open one of a suit or 2C. Add up your defensive tricks (A=1, AK=2, AQ=3/2, KQ=1, Kx=1/2, QJx=1/2). Multiply the total by 2. Add all length cards of more than 3 in a suit. If the total is 13 or more, open 2C.
For example, you hold: AKJxxxx KQJxx A void. Only 18 high card points, but what does the rule of 13 say? You have 2 defensive tricks in spades, one in hearts, and one in diamonds...total 4. Multiply by 2=8. Add length tricks (4 in spades and 2 in hearts)=14. Open 2C. If we change the hand slightly to AKJxxx KQJx Axx void, we still have the same 18 high card points, still the same void, still the same 4 defensive tricks, but now we only have 4 length tricks, so 4x2=8+4=12. Now this hand is not a 2C opener.

Rule of 15:
When deciding whether to open light in fourth position, count the number high card points and the number of spades, if the result is at least 15, you may open. This relies on the principle that you do not wish to open the bidding for the opponents. If you control the spade suit, adequately, they will not likely bid over you.

Rule of 16:
When deciding to raise a 1 NT opening to 3 NT, count the number of high card points and the number of cards 8 and greater. If the sum is greater than 16, you should raise to 3NT. This avoids the need for the use of 2NT as a bid showing 8 points and asking partner to raise if at the top of his 1NT bid. Accordingly, 2NT may be used for transfer or other purposes.

Rule of 17:
If partner opens a weak 2, add the number of high card points plus the number of trumps you hold. If the sum is at least 17, bid game in partner's suit. (Zeke Grabour, ACBL)

Rule of 20:
When deciding to open, you may open if the sum of your high card points and the number of cards in your two longest suits is at least 20. This rule is attributed to Marty Bergen.