1. Trading means simplifying: You want a simpler position when you are WINNING.
2. Trade queens if your opponent is attacking you.
3. If you are attacking don't trade queens. You might need her for checkmate!
4. Trade pieces if you are up material (and know you can win the endgame).
5. Trade PAWNS if you are down material. A lot of pawnless endgames are draws.
6. Don't trade active pieces for passive (inactive) ones.
The point is that it'll be easier to win an endgame with fewer pieces on the board, if you have an extra one!!
Here is a great example of a game where white has just won a piece. Next step? Simple! Trade...and simplify! Watch how it's done.
Now you try. In the following position, should white play Ng5 to try to trade on f7?
Here's what it would look like after trades on f7.
Who has more pieces developed? Black has 3 and white has....a pawn in the middle... Black's king is NOT in danger. The material is the same. Remember rule 6 above: Don't trade active pieces for passive ones.
How about this position? Should black trade? Think about who would be better afterward.