There's an easy fix, and even though there are several commercial products available under various names that may include words like: guard, net and even "thong", you can make one yourself with items you may already have around the house.
This Instructable will show how to turn an old arm warmer into a perfectly functional sweat guard. If you don't have an old arm warmer, you could use the "leg" cut from and old pair of tights or any other elastic material that will repel or deflect the sweat.
Step 1: Materials and Tools
1 x Arm warmer (or a length of similar elastic material)
3 x Velcro cables fasteners (I had some left over from a cabling project but a small package of straps costs ~$3)
The arm warmer actually wasn't in bad shape, but it's partner had been chewed on by our miniature dachshund and that arm warmer is the cover for the steadicam arm in my Instructable: Updated Bicycle mounted steadicam
Step 2: Assembly
For the front set of holes, with the arm warmer lying flat, line up the material and fold it over slightly to cut the first set of two(2) holes so that they line up top and bottom. Repeat for the second set of holes. The holes at the rear of the guard can be done as two small cuts on both sides of the material (see second image) as the rear Velcro strap will be passing through at a 90 degree angle, compared to the ones at the front. This will become evident when attaching the guard, since the handlebars are horizontal and the seat post is vertical.
Once all the holes are cut, thread the Velcro straps through the holes as shown in the picture. The Red and Blue straps are in the front and the Black Velcro strap is at the rear.
Step 3: Install the Sweat Guard
Now your ready to ride and if the guard gets particularly soaked with sweat it's easy to remove and throw in the washer (just remove the Velcro straps prior to washing, otherwise they will stick to other items in the wash.