The fastest way to reduce your sanity reliably is to eat Green Mushrooms, if you have them available. They can be found in Forests and the Savannah, can only be picked at Dusk and reduce your Sanity by 50 points, with no HP loss. With 1-2 raw Monster Meat pieces on top of that you'll become Insane in no time, just remember that Monster meat deals damage when eaten.
The only thing that gets close to that amount of Sanity loss is a Grue(a.k.a. the Night Monster) attack, that reduces your sanity by 20, but that is risky, since a couple of hit in short succession can easily kill you.
When you're done harvesting your Nightmare fuel, you can quickly gain Sanity by sleeping in a Tent: at a whooping 50 Sanity gained per use it will easily soothe your shaken nerves letting you finally get some good nights of sleep after facing your Nightmares. If you're having problems getting the Spider Silk required by tents, you can always create Straw Rolls, that let you get 33 Sanity per use.