Success at Forbidden Island is about setting priorities so that you know which actions to take and when to use special cards. It isn't always clear-cut, but in general you want the following priorities:

1. Shore up critical locations in danger of sinking.
2. Retrieve treasures for which you have four cards.
3. Trade cards to get sets of 4 treasures.
4. Shore up non-essential flooded tiles.

Shore up critical locations in danger of sinking. You lose the game if Fools' Landing sinks--or if both of the locations for any treasure sink before that treasure has been retrieved. These are critical locations and should be shored up, if possible. There are some situations in which flooded locations are not in immediate danger (when all three Waters Rise have been played and there are still at least two cards left in the treasure deck), but mostly any flooded locations are vulnerable to sinking.

Retrieve treasures for which you have four cards. You especially want to turn in sets of cards before the treasure deck gets reshuffled! The more cards you can return to the treasure deck, the more turns you will have at lower flood levels.

Trade cards to get sets of 4 treasures. Two pawns must be in the same location in order to transfer a card. If you want to receive a card from another player, it can be beneficial to position your pawn so that it is easy for them to reach you (or so that you're already co-located when their turn starts). Early in the game, when you hopefully have fewer critical threats, you'll likely want to have all the pawns clumped together to facilitate transfers. Sometimes the best player to give all the cards to is the one who could collect the treasure the soonest, rather than the player holding the most of them.

Shore up non-essential flooded tiles. Not all non-essential tiles are created equal. When you have the option, prefer shoring up tiles that maintain movement--especially to/from Fools' Landing. In general, the more tiles you keep from sinking, the more of a buffer your critical tiles have in the deck.

Many turns will see you wanting to accomplish more things than you have actions for. Sometimes a clever use of a Helicopter Lift or Sandbags will help you stretch your actions so that you can accomplish all or most of what you want. Usually this is a good idea! But beware of using the last of them for marginal return--you may want one for a real emergency! Sometimes using your special cards right before the treasure deck is reshuffled is helpful--it may help you squeeze out an extra turn or two at a lower flood level.