WyzAnt allows you to use their custom-built tutoring platform for online lessons. I have found this far superior to Skype, and comparable to many commercial, expensive web meeting platforms, and it's specifically designed for tutoring. While there are many features, and WyzAnt's materials cover them extensively, I want to point out something I've used that has greatly benefited my own use and, I hope, the experience of my students.
I own a Wacom Intuos graphics tablet, which I bought to annotate screencasts for a statistics app that I'm developing. A use case I didn't intend was online tutoring. WyzAnt's platform has a whiteboard with the ability to create objects, drop in files, add text, even Wolfram|Alpha output, to a screen both parties can see. However, the freehand drawing function looks very ugly if used with a mouse: it resembles a child's chicken scratch. Not exactly the professional look you want as a tutor.
Using a graphics tablet allows me to get natural looking writing as a I write on the whiteboard. While there are many ways to get good-looking whiteboard content, like typing your equations in the editor provided, it's much faster if you can just write naturally. If you plan to do a lot of online tutoring, it is an investment worth considering.
You can also use the freehand function on top of existing tools. For instance, I use the circle function to make the basic outlines of Venn Diagrams, and then fill in the details using my pen tool. Or if I'm tutoring Calculus, I might graph an example function, and then draw the tangent function, or sketch the derivative by hand.
The only downside is that your student is unlikely to have a device like this. That means if you want to see their work, you must have them either take a picture of it, show it to you on their webcam, or have them type. Since few students type equations often, they're unlikely to want to do that. Depending on the subject, this can be frustrating. One possibility is for the student to attempt problems on the their before the lesson, and scan them/send a picture of them. Then the tutor can add it to the whiteboard, and correct mistakes, ask questions, as the lesson progresses.
Online tutoring provides many challenges, but also opportunities for access to new markets. Using the best tools available sets you up to be as successful as possible. I highly recommend the use of a graphics tablet input device for professional-looking writing in online sessions.