I'm not sure if I'm doing things as efficiently as possible, but this is what we do (simple & not overly expensive)...
I have an unlocked iPhone that I use in England. A while back, I bought a pay as you go sims card from AT&T. Whenever we visit the US, I just log onto AT&T online & add minutes to the US sims card & put the sims in the iPhone. There are diff't PAYG plans.
If the trip is short, I put $25 on the card & choose the plan that deducts $2 a day any day that I use the phone (unlimited talk/text, but no data...I just use wifi). If I don't use the phone that day, there's no money deducted. If there's unused money on the acct when I go back to England, the balance stays on the acct for quite a few months. Either way, it's not a huge sum of money if I don't return to the US before it expires.
If the trip is long'ish, I pay $25 for 30 days of use (unlimited text, ~2 hrs of talk, no data). The phone number on the sims stays the same as long as I use it a couple of times a year. If too much time lapses b/w use, AT&T reassigns the phone number to another customer & I get a new temporary US phone number on that sims when I use it next (no biggie). For example, AT&T told me my current assigned PAYG phone number expires on June 12 if I don't use the phone b/w now & then. So, I can either let that number go OR I can contact them ~June 11 & ask them to extend the time it's assigned to me by a few months (they'll do it if you just ask....but they can't extend forever if you aren't using it, so you just have to negotiate). I can communicate with them via online chat on their website when I'm in England.
I don't have an ongoing AT&T phone plan. I just have an acct that lets me add money to the sims card as I need to.
I'm very dumb about Skype, but I do know enough to use it when I'm in the US for making 800 phone calls. That doesn't cost anything & I don't have to use my PAYG minutes. It's good when making mind numbing calls that involve being put on hold for 300 years.
I'm sure savvier people have better options, but that's what I do. It's nice to have that sims card all loaded up & ready to go when I land at the airport in the US. I think there's a gadget that lets you put two sims in one phone at the same time (so that it could still make/receive UK calls using your UK sims), but that's not really an issue for me.