Now that you have mastered your bidding system (You have haven't you!) you can move on to defense. This is a difficult task as the defenders can only see one hand whereas the declarer can see two. However good defense is extremely rewarding when you get it right. It is also very profitable as you will defend twice as often as you will declare; even more if you have a partner who hoggs the bidding. (Never!!)
The most important thing to remember is that this is a partnership game and you need to talk to them and decide what defensive signals you are going to use.
There are three types of signals, in order of priority.
This occurs on partners lead, especially the opening lead. You tell partner if you can help in this suit. The standard way is with a high card. This is called "High Encourage". Some people play "Low encourage" which means they will play a low card if they like your lead. Either way it is up to your partnership to decide.
Generally you give count on the opposition lead, A high card shows an even number of cards in that suit and a low card indicates an odd number. This is called Natural Count. Some people play Reverse Count which i the opposite, again, it is up to the partnership to decide.
3. Suit Preference.
This is the most beautiful of all. In some occasions it is possible to tell your partner which suit you would like. A high card says that you want the higher ranking suit. of the two most obvious suits.
Naturally which defensive system you play MUST be on your system card which should always be available to the opposition AND theirs should be available to you.
I found a great article on the web by Audrey Grant and it is available here.
She has written many books and has her own web page which is well worth a look