If you are an above average player it is possible to go infinite. This means you never have to buy gold but always continue to gain cards and gold.

The first step is to become an above average player. Even slightly above average will do. The best way is to play a lot of games, read the strategy forum and talk to experienced players. Most players are happy and willing to help out. Play against real people rather than the AI. Play against people with the highest rank and pay attention to their strategy and how they build their decks.

You will probably find you are better at one particular format, Drafts, Constructed or Sealed/Leagues. Then simply concentrate on that format. For each format there are constant tournaments that payout prizes to the winners. If you are above average you should win more gold in these than you spend in entry fees.