While you certainly want your trades to give you an advantage, you also want them to be fair.
The first trading tip is, once again, don’t be a jerk. People don’t like trading with jerks, and if you are constantly offering bad trades people aren’t going to want to trade with you at all. While you certainly want your trades to give you an advantage, you also want them to be fair. In Catan, as in life, trading should make everyone involved better off – although there are certainly ways to ensure that you get the better end of the deal. This doesn’t mean you can never ask for trades that are technically “unfair”, such as asking for a 2-for-1 trade. Just be careful not to do it too often, don’t be a jerk about it, and only do these types of trades if it still benefits both players.