I use t-mobile sims that I buy from local vendors. I mostly buy from simcorner.com who have always been very reliable. This recent trip I took advantage of an Ozbargain deal to buy from TravelSims Direct. They sent me a hand cut-down sim (micro -> nano) which didn't work and I had to buy a sim in the US (wasting time and money). Never again from TSD. The falling/fallen AUD means that US sims have gone up (
eg t-mobile 6GB 4G data, unlimited US calls/texts, unlimited calls to AU landlines, unlimited text to AU mobiles is $AU79 from simcorner.com. Buying the similar t-mobile "simple choice" plan in the US is US$75, or about AU$100).
I usually ask the sim card vendor to activate the sim 48-72 hours before I go (I never go for more than 25 days). They then email me the phone number (you can't get phone number till you activate). I can then tell friends and family the number, update US contact details for hotels, hire cars etc., before I go. When the family of 5 is going, it's much easier to sit around the dining room table programming each others' numbers into our phones rather than trying to do that once we're in the US.
Then, whilst over the Pacific, we install the sims into our phones and all phones are working as soon as we land in LAX.
I've never been one for GPS in the US because the road signage is infinitely superior to Victoria's, but on my last trip I had a couple of tricky addresses to find: Google Maps with voice-guided navigation was absolute heaven: I'm now a convert! Returning from Davis to SFO on I-80, Ms Google advised me of severe congestion ahead due to a traffic accident and suggested I-680 as an alternative; I did as I was told (it WAS a female voice and I'm married!) and had a dream run into SFO. Watching the late night news they showed an overturned semi that had completely blocked inbound I-80 for hours. Google Maps on data in the US is brilliant.
One tip, though..... if using your phone for GMaps, make sure you have either a cigarette lighter adapter or a powerbank as your phone may discharge rapidly. I use a Romoss powerbank because the last rental car I had last trip had non-functioning cigarette lighter sockets. The powerbank meant my phone battery discharged at about 3% per hour rather than 30% per hour and I wasn't reliant on the car providing power.
Finally, check that your phone will receive US frequencies: here's a table of AU & US frequencies as well as frequencies of common recent flagship smartphones:

or
https://dl.dropboxusercontent.com/u/83820413/mobile_phone_bands.jpg if the forum software mangles the table (I update this table when new phones are released).