I second Big John's endorsement of non-blended trifocals. I've used
them for about a year (and bifocals for seven years before that) and
have had no serious problems with glare or with having to position my
head just so. Just make sure that the lens segments are approximately
equal in height; mine are a bit skimpy on height in the middle
segment, which I'll have corrected the next time I buy glasses. I
didn't buy trifocal sunglasses; instead I bought lightweight non-
prescription clip-ons that use a spring-loaded expandable nose-bridge
and small end-clips to grip the lenses. They're lighter (and less
dorky) than the center-clip flip-ups, although reflections off the
front lenses can be bothersome when in bright sunlight from behind.
That's been a problem only under the bubble canopy of a friend's RV-8.