(disclaimer, Canadian here...)
US cash currency is usually well accepted in major cities in most "tourist" areas (with different exchange rate).
Use your credit card as you do normally.
Use your ATM card as you do normally.
I tend to have a reasonable amount of local currency with me just to get started; so I hit my exchange shop (or bank) before leaving; Canadian banks usually have US currency on hand, maybe the other way around is not true for most US banks having canadian currencies.
If I cannot exchange local currency before leaving, I hit the ATM at the most convenient time at my destination.
For example, I am driving to the US in a couple of weeks, I will get 200, 300 $US cash before leaving and use my CC for most paiement and use an ATM if I need more cash.