Trade when you think your opponent's piece is better than yours.
Don't trade your good or active pieces for your opponent's bad ones.
The numerical values of pieces is a guide,not written in stone. Sometimes a lowly pawn can have more value than a Queen.
if the game is "open" and there's lots of space on the board, you'd rather have your bishops. If the game is "closed" and the pawns are all locked tight, a knight would be preferable. Now think about the game you're playing... is it closed? Will it open up? Knights love "holes" i.e. squares that the opponents pawns can no longer control. Can you find a nice hole for your knight? Do you know which bishop is your bad bishop? (the one that is blocked by your own pawn chain) It's all about what piece is useful according to the dynamics of the position and how those dynamics might change....what is relatively static in the position and what is likely to change? good luck!
What you are talking about "suddenly losing another piece" I guess you mean the tactic of "removing the defender". Let's say, a pawn is guarded by a knight only, then first your opponent trades a bishop for your knight (which you recapture with a pawn) and suddenly your pawn, which was guarded by the knight before, is now hanging because the pawn with which you recaptured doesn't protect the other pawn like the knight did. This is something that ... well, one just has to learn to see :)
*stands in front of crowd* My name is Ladya79, and I can't exchange pieces well!
*crowd* Hi, Ladya79...
Here's my sad tale of woe: I know the values of the pieces, and so I know, all other things being equal, what pieces to trade for what (knight for bishop, rook for bishop and 2 pawns, a queen for nothing else but my opponent's queen). My trouble comes in when I'm trying to trade, say, a knight for a bishop and all of a sudden I lose one of my OTHER pieces that I'm really not counting on losing! :P Sometimes I'm only down a pawn, but I'm still down. It's hard to come back from.
So, when would you guys exchange pieces? When would you not? I need an intervention! I've taken the first step already:
1. I am powerless before the temptation to trade and end up -1, -3, -5, -8...!