Several years ago I doubled a very poor player in a five diamond contract that I had pushed her into.  The very sweet lady proceeded to make five diamonds.  The rest of the field played five diamonds, making six; or, three no-trumps, making four or five.  It was obvious that I had made a very basic mistake.   Since then I have developed a 'philosophy of bridge', (a few rules to bid and play by) based on certain truisms, which does not permit such bad bridge on my part.

Bridge Truism Number One:
Beginner and intermediate players are generous if left to their own devices.
Do not try to get good results from poor players.   Weaker players will give you very good results if you just leave them alone.   If you do things to throw them of their stride;   they will stumble onto the path of success.   Avoid balancing, preempting, doubling close contracts, doubling minor suit contracts, and most of all depending on their bidding when defending or declaring.

Bridge Truism Number Two:
Good players are very stingy and sneaky.
Good players will not only make you earn any good result you get from them but they will also try to talk you out of good results by using skill, deceptive carding and occasional deceptive bidding,.  Against good players you must use all of your tools which are not limited to:  aggressive balancing, aggressive preempting, and close doubles when they bid one too many.

Bridge Truism Number Three:
If a player become familiar with his opponents' strengths and weaknesses he can defend against or avoid the strengths and take advantage of the weaknesses.
If an opponent is an excellent declarer but a poor defender it might be a good idea to bid one more.   If a pairs defensive bidding is of questionable efficiency maybe it would be a good idea to open that hand you might otherwise have passed.   Give the opponent who is notorious for bidding too much in competitive auctions a little push.

If they will not make a penalty double of an overcall of a no-trump opening bid - they play negative doubles of overcall or 'stolen bid' and will not convert doubles to penalty doubles - stick a bid in frequently.  There are many ways to take advantage of an opponents' weakness and it is sometimes possible - not easy, but possible - to avoid an opponents' strengths. A player only need to be observant.

Bridge Truism Number Four:
Bridge players tend to be as consistent in their bidding and play as they are in their dress.
You can get an edge by taking advantage of an opponent's consistency.  If the player who will be leading against your possible no-trumps contract will not lead his/her own broken suit the requirement for a stopper in that suit can be shaded.  If the opponent who will be on lead tends to lead aces against slam contracts (any contract) you should bid the questionable slam (game).  Take advantage of an opponents carding if it is always or never true ( I once knew a player who was so consistent in her carding that she played high low with a doublton as declarer and I knew one who thought that playing a false card was cheating).  Players sometimes state how they defend:  'I never lead away from a jack against no-trump contracts' or 'I lead my shorter major against no-trump'.  If they are silly enough to tell you there secrets to success, you should be silly enough to take advantage of it when they defend against you.  Every player has his/her 'consistencies' you just need to be observant to learn what they are and clever to figure out how to take advantage of them.

Bridge Truism Number Five:
Occasional inconsistency can be a weapon; regular inconsistency will be a detriment to your partnership.
An occasional deceptive bid or play will keep the opponents guessing.   However frequent psychic bidding and strange defensive plays will destroy your game.  A player who is aware will find many opportunities for minor deceptive bids or plays which can help his/her game. Bidding diamonds with three small to avoid a diamond lead in no-trumps;  jumping to game after a simple raise with a minimum hand (sounding strong) to prevent a double or a successful part score contract by the opponents;  bidding a short suit before taking a save in your partners suit;  or, underleading an ace on opening lead in a suit contract are only a few possibilities.

It is important to be sure that you do not bid deceptively so often as to disrupt the game or so often that your partner becomes aware of what you are doing.  Both cases are contrary to good bridge ethics.

A thinking and observant player can get an edge in bridge.   The partner of a player may not take advantage of a hesitation;  but, his opponent may and should.  You must however be careful to not cross the line from being an observant player who takes advantage of when a player takes the card out of his hand before it is his turn to play if he has only small cards in the suit or plays faster than he should if he is trying to hide the fact that he has the ace - to becoming the cheat who watches to see if a player takes the end card from his hand.