For me, I simply use Bicubic Auto in PS CS6 to make the files larger of an existing file.
If I know I'm going larger from the start, I'll start in ACR, clean the noise at that point, add a little sharpening (usually the default), then set ACR to open at the maximum interpolated resolution. From what I understand, there's not too much difference between doing the up-res in ACR compared to doing it later on your master files, but YRMV, depending upon how much work you've done on the file.
The other way I do it is using Perfect Resize 7.x within PS. It has a whole bunch of other things that need tweaking as you go along and you need to work with it a little to tune it to your results. One thing is that it also can sharpen as part of the process.
In any case, from PS, you need to do your sharpening as the last step of the process before printing.
If you are not printing large on your own printer, to see the final results, simply crop an 8x10 piece of your final image and print it and then look at it on the wall.
Generally, when you print large, most will not view the image from 1-2 feet away like most on the LL board! That said, you can many times simply output the image from a lower-res image, 240 ppi, 180 ppi and even lower if it's really going large as in a billboard. The only way you will know what you are comfortable with is to print several samples at full size and then put them up. If you can't print and then show several large prints (I've got limited wall space my self), simply crop an 8x10 or 11x17 out of an image from 180 ppi at full size, 240 ppi, 300 ppi and 360 ppi and put each side-by-side on the wall and take a look. The results may surprise you!
The key to getting the IQ that will look great as a large print 30, 40, 60 inches wide from our DSLR cameras which the math tells us "never" to print larger than 11x17 or 16x24 according to its pixel density is to use excellent craft.
By excellent craft I mean, good technique at capture, the best lenses, clean sensor, tripod, cable release, critical aperture, etc. and then carefully working the file in ACR, LR, PS, etc. to get the best image. Then you need to edit ruthlessly and toss the images that are simply not the best. You've got to do a little pixel-peeping IMO, to see if the image has what it takes to go large. If you shoot JPEG, make sure sharpening is off since that creates image artifacts. Make sure you shoot enough images so that your best image isn't your only image!
Sloppy technique in your craft will probably means that it's a hit an miss with many more misses. Practice your craft!
I can tell you from experience that since I didn't write the rules in the number of pixels needed to make a print of a given size simply ignore them. You'll need to test to see what works for you, of course. My large prints are routinely from files substantially less than 300 ppi. They look just fine.
For a show currently at the National Steinbeck Center, Mission San Juan Bautista reVisions, several of the 36 photographs on exhibit were JPEG files from an iPhone, iPad, Nikon D100 (before I shot raw), Minolta 7, Canon G10 and other older/lower-res cameras. With the exception of maybe 3-4 images, nothing was captured with more than 10-12 MP. A few images were vertical crops from horizontal images and vice-versa. One of the iPad photos was printed from a cropped image and it's a 24x36. The smallest photo on display is perhaps 13x19. At the opening a last Friday, they passed the examination of our peers as I expected they would.
Close up, yes, you can see the difference between the images shot with the D800 and the D100, G10, iPad, etc. From a few feet away and considering the overall quality of the show, no. It really doesn't matter. Between the vision and the craft, the overall quality of the show holds its own.
You can see the book here: <http://www.magcloud.com/browse/issue/316049> and read about the exhibit here: <http://www.steinbeck.org/pages/3-photographers-30-images-san-juan-bautista-missio>
Even looking at these images closely, they pass the "18 inch" test. By that, I mean that they appear sharp and crisp looking at them closer than the general public.
In any case, there are many ways to craft your image to get large prints and each is valid. The only way to know what works for you is to try different techniques and see what will work best for you and your work.
