When I'm working on a digital illustration, there's a few things I need to have handy in order to get in a good groove. Speaking of groove, I like to listen to my favorite music or listen to tv when I work for background noise. Below are all the things I use when I work digitally.
1. A few sketches that I'm happy with.
Usually, I'll sketch whatever I want to draw on the computer on paper first. They don't have to be perfectly drawn, just a decent outline of the illustration. You can always change imperfections later. I'll take a photo or scan it in to place in Illustrator.
2. Adobe Illustrator and Photoshop
I live and breath Illustrator. Every illustration I do has spent some amount of time in Illustrator. The pen tool, blob brush tool, and pencil tool are my best friends. I use Photoshop to add textures with Kyle Webster brushes. My style is very colorful and bold with a lot of organic shapes. Illustrator helps me make my shapes and get my color palette perfected so that I can bring it in Photoshop and draw on top if that's what I'm doing for a particular illustration.
3. Wacom Tablet
This comes in handy when I'm in Photoshop adding textures or brush strokes. Also when I'm drawing type and want that "hand-drawn" look. My tablet is the Intuos Wacom Paper Edition. I love it, but I haven't used the paper functions yet. Kyle Webster's brushes work awesome with the tablet.
That's really all I ever use! As long as I have some time set aside to focus and maybe a nice coffee or tea, I'm golden. A bonus thing that is cool to have handy is a set of custom Illustrator brushes that I made. This is one of the cool things about my digital work. I don't need a bunch of materials and can take it just about anywhere to work.