Shown below are some options for tinting the top part of your helmet's visor, in case you don't want to go the way of wearing sunglasses or of a replacement smoke/mirrored visor.
The big advantage of these products is that with them you don't need sunglasses, and you can always have a clear view of the road, without tint, any time you want. It may sometimes take tilting your head a little to get the tint between your eyes and the sun, though you’ll usually want to look under it to watch the road.
The two items under discussion are the Helmet Sunblocker from Aerostich and the Fog City Speed Tint, available at CycleGear. They look very much alike but differ in the installation procedure. Costs are similar, in the $11-$15 range. They both have a separate style for Arai visors.
Both stick to the inside of the helmet visor, so they don't get goobered from flying crud. However, the Helmet Sunblocker is applied by using slightly soapy water to wet it, then sticking it into position and letting it dry there. The initial wetness lets you move it around a bit until you have the position you want. This is an advantage, as you don’t have to get it right the first time. They claim you can also remove it by re-wetting it, so it can be re-used, and indeed transferred from visor to visor, which is good if you go through two or three visors for a helmet before tossing it. You can also leave it in place for years without it budging. This involves some care when cleaning the inside of the visor.
The Speed Tint has a lip of adhesive along its top. You apply it by finding it and the visor’s center (left-right wise), poking it on there, then seeing how it lines up on the sides. You can then peel it off if necessary and adjust it by basically redoing the process. It's not too bad to peel off, but we all have good and bad days for removing tape-like things. Don't cut your nails too short right before doing this. You have to make sure you don’t get the Speed Tint on upside-down. The clue is that the part with the extra adhesive goo glares somewhat in the light. It lights up the top of the tint strip. Watch where the helmet's seal hits; it’s probably not a good idea to have it contact the top of this tint strip.
The Speed Tint's adhesive does let you peel it off and reposition it at first, but there's some indication that it is not really "re-usable" after several hours or so of setting. It is also somewhat sticky throughout its entire area, but there's a definite strip of extra adhesive along the top.
The Sunblocker is a lighter tint, though still fairly dark. Since it goes on with wet, soapy stuff, you can look through it and see basically what's in front of you, but stuff is a little distorted. You're not going to read street signs easily, but you should be able to see the cow on the side of the road and the car in front. You should probably only look right through it if you’re riding straight into a low sun and the street itself is all glare. Since there is no concentrated adhesive goo, the edges of the Sunblocker don't really light up.
For the Speed Tint, the vision through it is much less distorted. However, it is a much darker tint, and that fact makes reading signs and seeing some objects harder. This darker but clearer tint may be desirable to some. The Speed Tint's top edge does light up, but it's up high enough that it should be ignorable.
Both obviously had some thought put into their shapes, but the designers came to different conclusions, at least for HJC visors. The Speed Tint's shape seems to be a stronger attempt to have the edges more closely conform to the viewable area of the visor. Neither comes all the way to the sides, but the Speed Tint comes closer to "sealing" the left/right edges of the visor. In doing so, it also wings downward as you move to the sides.
The Sunblocker does that a little, but leaves a larger gap on the sides and along the top edge as you move to the sides as well. In a sense the Sunblocker plays it safer by not trying to cover all the way to the sides, where the Speed Tint leaves only about an inch on the left or right side of the view-through part of the visor. With its darker tint, the Speed Tint might block more peripheral vision, and on the test HJC shield it only left about an inch or so clear visor at the bottom sides (that's how far down it dips).
From the outside of the helmet, the Speed Tint has that mirror-y tint some sunglasses have and would hide the rider's eyes to an observer (if so tilted). In fact, they mention that you can wear it lower and have it easier to look through by default. On the other hand, the Sunblocker would only make it difficult, but not impossible, to watch the rider's eyes.
As for preference, the Helmet Sunblocker is good for those who like a dark, but not obstructing, tint. If you like more coverage and a very dark but clearer tint, the Speed Tint could work for you.
You can get a roll of Gila Static Cling window tint #JS248 from an auto parts store. It's meant to go in place on the top edge of the inside of a car windshield, but you can cut a strip to fit inside your visor. It holds up fine over time and isn't affected when you clean the visor.
To fit it, put the helmet on and stick a piece on the outside, adjusting it up and down until it's where you want it. Take the visor off and cut another piece to fit on the inside, aligned with the first piece on the outside. Then trim the edge of the cling film even with the visor edge. Remove the outside piece and save for later. That strip will do a dozen+ visors and cost less than $12.