Rest assured, it is not necessary to spend a thousand dollars on a portable tungsten sharpener (a fancy Dremel tool with diamond bit and angled channel) to get consistent results.
A few pointers to keep in mind...
Avoid using sanding belts or stones that are available for general shop use. These mediums will accumulate debris from other operations and embed that into your tungsten, contaminating your electrode and subsequently your work piece. Instead, dedicate a diamond grinding wheel specifically for use as an electrode sharpener and use it for nothing else. If you are patient, you can find deals on these wheels, such as this... Enco diamond wheel
When sharpening an electrode, the striations left behind affect the ease with which the electrical current will travel down to the work piece, so the more you can keep those striations traveling along the length of the electrode (parallel) the more low current and start control of the arc you will retain. This is the primary reason it is ill advised to use the side of a grinding wheel with the electrode perpendicular to it's position, as it creates lateral rings that will inhibit the flow of the current.
Always wipe down your electrode with a bit of alcohol or acetone before placing it back into the torch to keep your electrical interface as tidy as possible.
Here's a quick video I put up to illustrate the process I use...