Cryxis, one other thing to think about. While giving them valuable furniture is helpful, they prefer a masterwork chair made of rock over a crappily built gold throne. Specialize your furniture folks.
In general, as long as you don't do something nasty to the dwarves like have them dance with the undead, the following will keep your dorfs in a very happy to just below ecstatic state:
Decent bedrooms.
Legendary Dining room (easy with masterwork tables/chairs)
A variety of goods in your meals.
A variety of booze.
Make sure you're using a current copy of DFHack to avoid the military happiness bug about patrols.
Make sure you stay up on your clothing creation, and specialize a few clothesmakers so they're producing high end goods.
That's it. Other than massive deaths of the military you shouldn't have significant issues.
Now, if you REALLY want to boost things up...
Train up a metalworker and have him eventually able to create masterwork statues, create a statue garden.
Use masterwork doors along the well traveled routes, can really spruce them up.
Create a waterfall. They're relatively easy, if a bit time consuming, if you have any form of water source on your map. Mist + clean dwarves. Stick it in front of the dining hall, with the bedrooms on the other side.
Specialize a few dwarves in engravings. Go for the ones that 'admire tradition' so you can avoid 4000 engravings of cheese... or roaches. First have 'em smooth everything, then have 'em engrave everything.
Create a 'zoo'. Be careful with this. What one dwarf loves, another can hate... sticking the 'zoo' in their personal bedroom is helpful.
Once the spiral starts, though, it's tough to stop the momentum. You need to isolate the causes and get them (if possible) out of sight. And leave them there. Entomb them in rooms so no one finds the body, engrave a slab. Send entire families out to pick berries on the surface... and seal the front door. Find dwarves who are either new, or 'don't care about anything anymore', and burrow them in a safety zone, away from the soon to be psychopaths.