All the other answers are really great, so I just want to add:

Don't buy the Copper.

Copper cost 0 to buy; if you draw a dead hand (all victory points, for example), there's absolutely no shame in skipping your turn.

Also, I've seen people with multiple buys for the turn spent all their money on their first buy, and then take a free Copper. Unless you happen to be playing with Gardens ($4 card, worth 1 VP for each 10 cards in your deck) and want to artificially bloat your deck, don't buy the Copper.

All the other answers are really great, so I just want to add:

Don't buy the Copper.

Copper cost 0 to buy; if you draw a dead hand (all victory points, for example), there's absolutely no shame in skipping your turn. It will ultimately end up bloating your deck and make it more likely you draw a $1 card instead of something more useful.

Also, I've seen people with multiple buys for the turn spent all their money on their first buy, and then take a free Copper.

Unless you happen to be playing with Gardens ($4 card, worth 1 VP for each 10 cards in your deck) and want to artificially bloat your deck for the chance at an extra VP, don't buy the Copper.