Did anyone else read 'tricks for enlarging penis' at first glance .. nope? just me then.
OT: there is no way definitive way to do this as it depends entirely on the image you are trying to upscale.. you may be able to use content-aware scale if it;s not too complex... but if it is then you may get strange results. Another option is to increase it and then apply some subtle blurring to smooth off the jagged edges and maybe try a smart sharpen folder or something..
Also try the biocubic smoother, if you go to image, resize, under resample click biocubic smoother. increase it a few % each time.
if all else fails try http://www.alienskin.com/blowup/ i've not used it but alienskin are good.
In short; it will be hard to keep the exact same level of detail because you can't add detail what isn't there.