I have horrible myopia (near-sighted) and have looked into this since I can't focus manually worth a darn.
If your vision problem is either mild near or far sightedness, you can just adjust the knob (commonly referred to as diopter adjustment) on the camera as previously mentioned and look through the viewfinder without your glasses.
If your vision problem is more pronounced (me) and cannot be corrected with the adjustment on the camera (mentioned in earlier post), you can buy supplemental vision correction lenses. They are named and classified differently depending on the brand of camera you have, but Canon's are called "dioptric adjustment lenses". They just clip on the regular eyepiece and provide additional correction to what is already there. If you decide to do this, read up on it because Canon has a weird way of defining how much correction they provide which you have to understand and you also have to know your prescription to get the right one.
INFO
You can buy third party focusing screens for most cameras that are brighter and have different types of focusing methods, some of which are easier to use when manually focusing.
Or, you can do what I have done. When possible, just use the Live View feature if your cam has it, zoom in and focus while looking at the LCD. My macro shots have improved greatly using this technique, but it won't work for everything - kinda tough on birds in flight.
