The best lenses and frames to use depend a lot on how bad your eyesight is. If you have a relatively weak prescription, then the best lenses are glass, and the best frames are the teardrop kind which give you the best periferal vision. Good quality nylon frames of this sort are available from REI and Mountain Equipment co-op for about $15. There are a number of good glass lenses available, with some of the best being photo-sensitive lenses which darken in bright sunlight. If you get these, make sure you get the darkest ones possible, and you may also consider a mirror coat or the like to darken them even further if you spend much time above tree-line or on glaciers.
If you are like me, and have really bad eyesight and a really strong prescription, then glass lenses in large teardrop frames are too heavy and uncomfortable, and tend to slip down on your nose because of the excessive weight. The two things which can be done to combat this excess weight is to get a lighter lens and a smaller frame. Round frames have smaller lenses than the traditional tear-drop shape, and still provide adequate vision. MEC sells a round nylon glacier frame with adjustable, interchangable components (about $15) and some of the high-end glacier glasses companies such as Julbo and Cebe sell round metal frames which are perhaps a little sturdier, slightly better ventilated, and a little more adjustable on the fit) The metal frames are considerably more expensive. (about $50-100)
There are a number of choices for lightweight lenses. The cheapest option is the generic "optical plastic" or "CR39" as the opticians call it. It is light, strong and relatively inexpensive, and is pretty much universally available at any optician. You can get CR39 lenses tinted in pretty much any shade you want. The drawbacks of CR39 are that they tend to be a bit thicker than some other materials (which offsets the light weight somewhat), and, they do not hold dark tints all that well. If you use CR39 lenses in bright sunlight a lot, you will notice that the tint gets lighter and lighter. The darker the tint, the more you lose. Ultimately, the glasses which were quite dark when you bought them end up not dark enough for really bright conditions.
You can get around the drawbacks of CR39 by getting more advanced (and much more expensive) lenses. A "high index" lens is thinner and lighter than a normal CR39 lens. To solve the problem of tinting a CR39 lens, you can get a high-tech laminate or sandwich lens, which has 2 high index sheets with a very thin a layer of tinted, polarized glass in between. These lenses are permanently tinted and will not bleach out as much as will regular CR39 lenses. In addition, the polarizing effect gives sharper vision in high-glare envirionments such as snow and water.
On any glacier glasses, be sure to get UV, anti refraction, and anti scratch coatings.
Over the years, I have used all of the sunglasses described above. What I have finally settled on are a pair of round, metal glacier glass frames with a high index polarized sandwich lens. They are durable, light weight, comfortable, and perform well for extended trips on glaciers and snow fields. The polarization is a nice bonus, because it makes them very useful for fly fishing (easier to see into the water) as well as giving me extra sharpness when climbing and skiing.
I spent almost $300 on these glasses, but they use the most expensive components. If you buy the
frames at MEC or REI, you can get a decent pair of sunglasses with CR39 lenses for a total cost
of about $70 including the tint, UV, refraction, and scratch coatings by taking the frames to a
discount optician. If you spend a lot of time in bright sun, you may have to go back to get them
re-tinted every so often, but that extra trouble may be worth the money savings.