This is one of those posts that could save you a lot of money. I learned most of this the hard way, so you don't have to.
Don't Use Your Regular Bank
Your US bank will murder you on exchange rates and fees. I'm talking 3-5% above the mid-market rate, plus a flat fee, plus a foreign transaction fee. On a $1,000 transfer you could lose $50-80 without realizing it.
Use Transferwise (Now Wise)
By far the best option I found. They use the mid-market exchange rate and charge a small transparent fee. On a $1,000 transfer I saved about $40 compared to my bank. Over a year that adds up to hundreds of dollars.
The ATM Game in Bali
BCA ATMs have the best rates and lowest fees of the local banks. Avoid the standalone ATMs near tourist areas — they will offer to do the conversion for you at a terrible rate. Always say NO to "conversion" and let your card do the conversion at home. Withdraw in large amounts to minimize per-transaction fees.
Keep a Buffer
ATMs sometimes run out of cash, especially on holidays. Keep a week's worth of rupiah on hand. The rupiah comes in massive bills — a 100,000 IDR note is worth about $7 — so your wallet will always feel full even when it isn't.