General Chip Placement
Chip placement determines the winner of Sequence, so choosing spaces that maximize your opportunities to win is critical. Ideally, you should group your spaces tightly together near the center of the board. Placing your chips in the center provides two valuable advantages. First, chips which are placed together can be used in multiple directions at once. For example, a single line of chips can only grow in two directions (at each end), but a square of chips (four placed together in a square) allows growth in 12 directions. Playing your chips near the center also ensures that your lines have plenty of room to grow in both directions, rather than being stuck at the edge without enough spaces.
Re-evaluating the Board
Re-evaluate the opportunities you see after every play that your opponents make. Every play changes the dynamics of the game, and each new chip on the board creates both opportunities and obstacles to your success. You must re-examine each of your potential five chip plays to be sure they're still viable options and haven't been blocked. Closely examine the spaces your opponents play to gauge how close they may be to successfully creating a sequence. While you must plan ahead to win Sequence, you shouldn't become so focused on your original strategy that you miss new opportunities to score.
Counting Cards
Each Sequence game consists of two decks of playing cards, meaning that there are two of each card in the deck. As you begin to make plays and plan where you might score a sequence, you should always be aware of the specific cards needed to succeed and how many of each card, if any, has been played. Waiting for a specific card when both its cards have been played would be fatal to your strategy. Be mindful of the cards you need to win as well as the cards that your opponents are likely waiting for to win, as well as the number of wild cards played versus those left in the deck.
Hoarding Cards
In Sequence the most powerful cards in the deck are the jacks. The two-eyed jacks are wild cards which can be played anywhere, while the one-eyed jacks allow you to remove any chip from the board. Because they are so powerful, it's best to use them only in emergencies: either when you're about to score a five chip sequence, or when you're about to lose the game because your opponent will score.