Most of the game revolves around trading as it is the most effective way to get beans out of your hand and onto the board. There are a few subtler aspects to the trade system that it's worth pointing out.

"Free Bean" Trades

There is a balance between offering trades that benefit you and offering trades that benefit the person your trading with. People are more likely to trade with you if you're offering something they want, but sometimes the thing they want is just to get rid of a card that doesn't fit their fields. Don't be afraid to ask for a bean without offering any particular bean from your hand (especially as the active trader). Your trade partner can add the bean they want when they propose a counter offer. Likewise don't be afraid to give away a bean for free. Sometimes (such as when it's your first card and you would have to harvest to play it, and your next two cards fit onto your fields perfectly) it is more valuable to simply not have the bean in your hand, or on the trade as the case may be.

"Same Bean" Trades

Trading a blue-bean for a blue-bean seems useless at first, but it allows both you and the player you're trading with to plant those beans right away. It can be a useful way to get beans from later in your hand into your fields before having to harvest to make room for a new bean.

"Final Round" Trades

Similarly, due to the number of beans in hand and in the gold piles of the various players, the final round is usually much shorter than the first few rounds. It can be quite useful to make more liberal trades late in the game if it helps you get that one extra coin in your field.