How to play Gin rummy well once you know the rules: the decisions that matter, and the habits that win.
Kings, queens and jacks count 10 each when they are not in a meld, and they are the hardest cards to meld because they have fewer neighbours to combine with. In the first few turns, discard high cards that are not already part of a pair or a run.
Low cards are cheap to hold. An ace or a two costs almost nothing if you get caught with it, and it keeps your deadwood low enough to knock quickly.
Picking up the top discard tells your opponent exactly which card you want. Take it when it completes a meld or clearly improves one, and otherwise draw from the stock, which gives nothing away.
Your opponent's choices tell you just as much. A card they pick up is one they are building around; a card they discard tells you what they are not collecting, and which cards near it are safer for you to throw.
Every discard is a potential gift. The safest discards are cards of a rank your opponent has already thrown away, and cards adjacent to what they have discarded in the same suit. The most dangerous are cards close to what they have picked up.
Late in a hand, giving up a little of your own progress to avoid feeding your opponent is usually the right trade.
You may knock with 10 or less in deadwood, and early in a hand a knock with a low count is often worth more than waiting for gin. Your opponent has had few turns to reduce their own deadwood, and every turn you wait gives them another.
Late in a hand, the danger turns around. Your opponent's deadwood is probably low too, and knocking at 8 or 10 invites an undercut, which pays them the difference plus 25. The longer the hand has gone, the lower your count should be before you knock.
Three sevens can only grow one way. A run of five, six and seven of hearts can grow at either end. When you have a choice between cards to keep, keep the combinations with the most ways to improve.
Remember that when you knock, your opponent may lay cards off on your melds. Showing an open-ended run gives them a place to dump deadwood; knocking with sets that are already complete gives them less.