"Perfect Resize" in adobe photoshop is really the best way to do it. It uses fractals to blend pixels together and kind of spoofs higher resolution through interpolation. There are other pieces of software (both paid and freeware) that are more focused on this and do it for you, but if you already have photoshop, I can't really see any reason why you'd need to download something different since this feature works quite well.
My dad has this old point and shoot camera that takes pictures in 72ppi (which is atrocious by todays standards), but he loves the damn thing and its easy for him to use so he uses it regularly. I've made several of his pictures large enough to actually use via photoshop and perfect resize.