Almost every time I create an illustration, I use Photoshop to put several pieces together.
I start off by making some pencil or ink drawings that I scan and open up in Photoshop. Thereafter, I experiment and explore what textures and drawings work together. By putting everything on different layers, I can control what I need to add, remove or edit. This method saves a lot of time and most importantly, it's great fun following the process of constructing the final illustration.