Many (all) of the suggestions I usually offer have been made already. So, just to consolidate them:
Drop down visor - many brands offer this option, I love my LS2 with it. You can get higher/lower tint for many of them
More-tinted visor - keep a spare for after sunset if it's very dark. IMO Shoei has the best OEM tinted/mirrored shields but they're not super dark. On the other hand, my
Nitek's visor was so dark that I practically couldn't use it on a cloudy day
POLARIZED sunglasses - Shouted for emphasis... I'll never go back to non-polarized glasses after trying them out. Never realized how much glare is everywhere until I bought my first pair. It was like, all of a sudden, the world is in HD when it's morning/evening. I will never buy another pair of sunglasses that's not polarized.
Strip of tint on the top of the visor -
These worked well the one time I tried them, but I typically go with a mirrored visor and it didn't look right together. Alternatively you could use car window tint that's really dark, like 3M limo tint
Strip of tape on the top/bottom of the visor - cheaper/easier than above, somewhat effective trick
ADV/Sumo helmet - I loved the Icon Variant's peak for sun blocking. Unfortunately they're usually pretty heavy for longer street rides
Tinted windscreen - Something like
these could work, depending on the bike, you could add/upgrade a tinted windscreen that you can crouch/tuck behind while facing the sun