I agree that using an ATM for needed cash is your best bet, but you must be careful about WHAT ATMs you use:
The bank-affiliated ones are generally safe, the one in the Farmacia Guadalajara is good; the one at PVNet in South Side is safe; any well lit inside unit in a larger store, and any secure unit should be OK (ie, it requires your ATM slid through to open the door, and the doors are in working order).

You should avoid using your Credit Cards unless absolutely neccessary; any items you can arrange to be billed ahead of time on your card like the room, a car, etc, would be wise to do so you can monitor the card activity before leaving.
PV has an ongoing problem with cloning, running the card through a tiny scanner in the pocket on the way to charging it with legitimate charges; this lifts the mag strip data and creates a new card pronto that is used and billed to you.
Yes, you can get the charges reversed and the card cancelled and replaced, but ID theft is a nightmare no one wants to tackle, trust me I know...

I would bring only a few hundred dollars in US currency for emergency usage.
Although some places will take US dollars, the exchange rate is not very good, and they are stuck with going to the bank and standing in endless lines to exchange them-it's not like when the inflation rate was high and US$ had value to them.

Invest in a security pocket or have a pocket it is very hard to pick from if you are carrying an old-fashioned wallet (never a back pocket).
Theft is rarer in PV than some areas, but the summer brings National vacationers and more theft is prevalent during that timeframe unfortunately...

PS don't forget to call your Bank, Credit Card Cos and any other relevant financial institutions and let them know you will be out of the country and using the card in question- having your cash flow suddenly dry up because they think it's stolen and your not returning their messages on your home phone system is a BAD THING...

Good Luck and enjoy a very nice place to visit,
Seaneen