Tip 21
Rainbow

In pots contested between you and only one other player, often the flop helps neither player.  Therefore, whoever makes the first bet often wins the hand.  Always be on the lookout for flops that contain few or no likely draws, as these boards are more likely to be of little interest to your opponent.

  A typical such board is A-9-3 rainbow*.  Since this flop has no straight or flush draws, and the presence of an ace means that there are no overcards to chase, it is extremely difficult for your opponent to stay in the hand without a pair.  Furthermore, if his pair is smaller than aces he may give up if you bet, since an ace is a likely card for you to be holding.

  So, it is imperative that you play these situations aggressively, particularly if your opponent only called before the flop.  If he had raised, that would show a real interest in the hand, and you would probably have to show him the best hand to win.  However, if

*Rainbow: Of all different suits.

he merely called preflop, then it is often correct to bet if you are first, regardless of what two cards you happen to be holding.  The chances are great that he will fold.  If you get called, no rule says that you must continue your bluff on Poker Turn.
  When your opponent is first to act and he checks, you must bet with this board.  Again, it is very likely that your opponent is holding a had like K-Q or 5-5.  A-9-3 is a poor flop for these hands, and you should bet when he checks, even if you have nothing.  There is free money in the center of the table, and you can make one small bet to lay claim to it.
  On the other hand, what should you do when your opponent bets at this flop and you are next to act?  It is good to have at least some knowledge of your opponent here, as some players bet only when they have an ace, whereas others bet lots of hands (perhaps themselves trying to pick up the pot).  All things considered, here are some general guidelines for how to respond to a bet:

Your hand

Your action

Middle pair  
call
A-x  
call
A-Q/ A-K 
raise
A-A, 9-9, 3-3
call - and bet or raise on Poker Turn
A-9, A-3    
call - and bet or raise on Poker Turn