Tactics
From Polarity
A definition of Tactics: Tactics are the specific means in which to achieve short term goals, from a specific situation to specific situation basis. Distinct from Strategy in that it does not concern itself with the overall goal of the game but rather short term means of moving towards that goal within a strategic framework.
Impossible Captures
Basic Method
Here is one simple way of creating an impossible capture. Say there are two opposing foundations a fairly short distance appart (say 1.5 disc diameter). Lay a low standing piece facing directly toward the other foundation. Now your opponent then places a leaner on his foundation on the far side away from your leaner. You then simply drop your leaner so that it touches your opponent's foundation without disturbing his leaner. He now has an impossible capture to deal with. Your opponent should attempt to capture it so his standing piece falls down with his color up to minimize the damage. The board after his failed capture will have two foundations of each color again but farther apart. This results in a net gain of a piece (and thus a negative point) for your opponent. Also, because you opponent loses their turn, you have really gained 2 points on your opponent after this manouver (more if the impossible capture involved a tower), because you then go on to play a piece.
Alternatively it is possible to simply drop your action piece when it is your turn so that it contacts the foundation/tower in question. This is a net gain of three or more points for you because you also played a piece from your stack. (It is also your turn to play a standing piece!) (Note that this method is legally questionable and could lead to abuse.)
For greater deception or when your opponent gets used to seeing this coming you can try rolling standing pieces to the correct position or to directly contact his foundation/tower.
Defending Against
First get used to looking for the above situation on the board (your opponent has a foundation close to a foundation of your own with a leaner on). For better defence try to place your leaner to one side or close to your opponent's piece (not opposite) so as to make this tactic less viable. This way your leaner should snap up and make a nice tower for you, or give your opponent pieces in his stack.