Photography Accessories Lighting

Wat are some inexpensive accessories every photographer should have? (READ DETAILS)?
I'm currently a student studying theatre, so I don't have a whole lot of money to spend on photography equipment! However, it is a way to make money (headshots, senior portraits, etc.). I want to take my photography to the next level, though. I currently have a Canon Digital Rebel XTi. What are some lenses, lighting equipments, flashes, etc. that would be helpful? Thanks!
50mm f/1.8. $100 lens. Waaaaaay better than the crappy 18-55mm that came with your XTi.
External flash if you want professional-quality portraits. These are not cheap, unfortunately. If you're really fancy, you may be able to pull it off with a reflector instead. But it's tricky.
But the number one most helpful thing for your photography, if you don't already have it, is the least expensive thing on this list. Spend a few hours online learning how to shoot in Manual. Once you understand the relationship between your photos and aperture, shutter speed, ISO, etc. then you'll actually know what you're doing instead of letting the camera do it all for you (and the camera gets it wrong).
Good luck!
Broad and Short Lighting in Portrait Photography