Visors are traditionally the preserve of the pros but the inexpensive, universal fit Shadow Visor is looking to change that.
Made from tinted polycarbonate, it will fit most standard road helmets. Fitting is relatively straightforward but don't rush it - you only have one chance to get the strips on right as the adhesive sticks instantly.
First wash the helmet using soapy water and dry thoroughly. Offer up the backing tape against the helmet, ensuring it's equal on both sides (it might be helpful to mark the helmet with a pen as a reference). Remove the backing very slowly, as you would when fitting bartape, and smooth against the helmet until it is evenly applied.
The visor simply Velcros in place and is easily put on and taken off. If you use glasses, the visor takes a few minutes to get used to, but the large surface area is very effective, keeping the sun's glare and airborne particles at bay.
My initial reservations concerning the security of the Velcro and the whippy characteristics of the polycarbonate proved unfounded when on the bike. But the limitations become apparent in the rain where wiping it clear could be difficult.