How to play Spades well once you know the rules: the decisions that matter, and the habits that win.
Count sure tricks, not hopeful ones. An ace is almost always a trick, a king usually is if the suit has at least one other card to protect it, and a queen only rarely. The ace of spades is a sure trick, and long spades are worth tricks on their own: a fifth or sixth spade is usually one more.
Shortness in a side suit counts too, if you have spades to trump with. A void or singleton with three or more spades is often worth an extra trick that no single high card would show.
Each trick over the contract is a bag, and ten bags cost 100 points. Underbidding for safety feels comfortable until it takes a hundred points off the partnership several hands later.
When your side has made its contract, stop taking extra tricks you do not need. Duck low, give the lead away, and when the opponents are close to their own bag limit, a trick or two that lands on them is a trick well spent.
A nil wins 100 if you take no tricks and loses 100 if you take even one. Bid it with low cards in every suit, few or no spades above the nine, and no aces or kings you cannot get rid of. A single high spade is usually enough to break it.
When your partner bids nil, your job changes. Win tricks they might otherwise take, cover their high cards, and lead suits where they are short so they can discard. Your own tricks still count toward your bid.
Do not trump a trick your partner is already winning, and do not overtake their winner with a higher card of the same suit. It wastes a trick and may create a bag.
Leading a suit your partner is short in lets them trump. Leading back the suit your partner led first helps them cash their high cards. The partnership's contract is shared, so a trick won by either of you counts the same.
When the opponents have bid high, the tricks that stop them making it are worth far more than your own overtricks, since a missed contract loses them the whole bid. Play to beat their bid, even if it means taking bags.
Count the spades played. There are thirteen, and knowing how many are left, and who is likely to be out of them, tells you when your side's high cards in other suits are safe to play.