Take a tile from the discard pile only when it directly completes or clearly advances a run or set in your hand.

Drawing from the discard reveals information, because opponents see exactly which tile you needed. Weigh that cost against the benefit: grabbing a tile that finishes a group is usually worth it, but taking a marginal tile just to fill your rack can tip rivals off to your plan. When in doubt, draw the hidden tile from the pile instead to keep your hand a secret.