Here's a summary of generally accepted methods to kill yourself easily and painlessly. This list is incomplete, and may be updated. For discussion visit /suicide/ itself or some of the IRC channels: #8c/suicide/ and #suicide at Rizon.
This document is released into the public domain. Last updated 2015-05-24.
DISCLAIMER: YOU ARE RESPONSIBLE OF YOUR OWN LIFE. IF YOU GET BRAIN DAMAGE FROM TRYING THESE GUIDES IT'S YOUR FAULT. DO THE RESEARCH!
With firearms and drop hanging, your objective is to break your spinal cord. That way blood circulation and signals to and from the body stops instantly, and brain shuts down. Hanging with partial suspension you only need to block circulation, while keeping airways open.
Breathing gases, you want to replace oxygen. Difference to suffocating is that there's no carbon dioxide buildup, and you'll panic less.
Your best bet is a 12 or 10 gauge shotgun or something as powerful. Handguns or other small firearms do less damage and the chance of survival is greater.Always aim through mouth, barrel up, gun secured.
There are people who swear to this method, but there are some problems with it too. Painless if done correctly. Low risk of affecting nearby people.
Argon or any other noble gas works equally well as a substitute.
Notorious gas that is fairly easy to produce. Does not induce pain itself - smoke and carbon monoxide are bigger concerns. Equal chance of success than in helium if done correctly.
Propane should work as a substitute, though there's an added fire hazard.
These methods are not recommended and may have low success rates or induce pain before death.
Overdose or poisoning
If you can obtain heroin or other strong opiates, shoot those. Combine with alcohol. Don't experiment with them to prevent tolerance.Barbituates (e.g. Nembutal) should work too.
If you try something else, you'll most likely survive or die from organ failure. That's not nice.
Jumping
Jump from something really high. Otherwise it's really a hit or miss.
Cutting
Seriously, don't be a fucking emo. It's not that effective either. You need to cut very deep, which is hard.
Electricity
Make sure it's high voltage and that it goes through you heart or brain. Pretty painful.
Drowning or suffocation
More painful than the gas methods described above.
Explosives
Try this only if you're an expert on the subject. Do not try to make your own!