You can spin the Wheel of Fortune to earn shields, but elsewhere, they'll cost you a lot of money. Get good at working the wheel so it gives you what you want (at a cost of 200 coins per spin), but otherwise you'll want to spend coins investing in power-ups like Coin Addict and Magnet. Level those up a lot to make it easier to get additional shields, and once you have them, make sure that you're only using them when they can do the most good. If you spend them early in a run, there's no going back even if that run quickly goes sour.