True. It’s image dependent and dependent on the look you want to achieve and VERY time consuming to get the best possible results.
One method that works some of the time for me is to resample by 102% (thats a 2% increase in size) then apply an unsharp mask, and repeat. As you can see to double the image size your doing this procedure about 35 times, but it’s one of the only way to get an acceptable result. You can then retouch the final image. This is usually only worth while if it’s a one of a kind image that you cannot attain any other way than the resolution you have and need it larger.