Shoot 'em with a Colt .45.
(Of course, this assumes you’re the Saint of Killers and all, but no plan’s perfect.)
In American Gods, gods died because no one believed in them.
in the Belgariad Garion killed some evil god by being possessed by a god.
God, I can’t believe I brought up David Eddings…that’s a blast from the past.
Get yourself a good Morganti dagger, wrap a semi-sentient chain made of gold phoenix-stone around it, and persuade a nice Issola lady to take up residence in it. Then plant it between the god’s shoulder blades.
Alternatively, you could take the approach advocated by others here, and wreck the faith of the god’s worshippers. The theocide Jame, in P.C. Hodgell’s Godstalk, does this by systematically interfering with an already weakened sect’s ceremonies until everyone gives up in disgust. Of course, she only did it as an experiment, and resurrected the god later.
Couple that I haven’t seen mentioned:
Mistletoe spear. Unfortunately this only works on Baldur and the effect seems to be temporary.
If you live in the Fred Saberhagen universe, try to get your hands on Farslayer or one of its mates.
Man, nobody’s noted the method of the best god killer of them all: Xena! Of course, her tactic was to get blessed by God so she could kill gods, and then, well…kill them in anyway possible.
Apparently, the bones of Uranus will make handy weapons as well, according to Hurcules.
Norse gods could pretty much die just by beating the ever living tar out of them. Thor’s destined to be poisoned, but everyone else is pretty much destined to fall on a spear or axe or lance or whatever their opponant has. Nothing too terribly fancy, just fitting.
Being torn apart seems to be a very common method.
The big problem, though, isn’t killing a god…it’s making sure they stay dead.
All you need is the blood of a Golden Hind, right?
I may be misremembering but I thought she was actually blessed by the series’ Jesus stand-in, Eli. She could also take out angels, so it seems odd that cap-g God would give her the power to do that.
Not quite; they slowly got weaker when people stopped believing in them, and some of them eventually withered away as a result. Many of the gods, though, either went insane or committed suicide before that happened.
Great novel, though. Definitely recommended.
Ask Nietzsche.
My preferred method is collapsing a black hole even further, sending its matter through the near-infinite layers of reality, sucking a good portion of our space/time into another universe’s space/time, and hope that it takes the god with it.
'Course, that creates a bit of a rift in our space/time, so the universe collapses over the rift. This looks a lot like a gravitational pull. The last time it happened, it created what we now call “The Great Attractor”.
Might I recommend reading The Jehovah Contract by Victor Koman? Just what you’re looking for: an assassin in the near future is hired by a televangelist to kill God/Jehovah/Allah. Interesting book if you can find a copy.
Or, as another poster said, just find yourself a Babelfish, logically think your way through how it could have possibly evolved, and God disappears in a puff of logic.
Q once got bored and offed himself… that would take time though and has little possibility for your input.
OTOH, according to many billboard toting ravers (not the ones that glow in the dark), God is already dead, so you’re wasting your time.
IIRC, some Greek gods (especially the early ones) ended up semi-permanently dead, though usually at the hands of their more powerful children, so unless you’re a somewhat clueless deity with a broadband connection I’d say this one’s out. 
You are misremembering. Xena got her blessing from the God of Eli, who was the Jesus stand-in. Her blessing came something like, 25 years after his death, when her now grown up skank-ho, I mean daughter, got baptized and God’s messenger appeared to her and granted her the power.
And later on, she tried to kill the Archangel Gabriel, but he just laughed at her and God took her powers away. She then had to convince Caligula who had become a god (?) to kill himself.
So, apparently suicide works as well. There are also plenty of stories out there with weapons that can kill a god, but it always seems to be “a” god, and not “the” god, so if you’re heading for the Head Honcho, you may have a bit of a problem.
Again, I’d go with the “rend them limb from limb” method. Seems to be the most efective.
Use Shieldbreaker if the god is manifested and characteristically uses a weapons of his/her own (trident, spear, bow, hammer, sword, thunderbolt, etc.) If you cannot contrive to meet the god in face-to-face combaat, use Farslayer.