Inferno mode is available for level 60 players and is also the highest possible difficulty in Diablo 3. Anyone who has tried it will tell you just how hard it can be, especially if you don't know what you're doing. Inferno mode was actually harder before Patch 1.0.3, but it is still hard even now after the nerf.

Here are my tips to having a successful Inferno run:

Avoid Impossible Elites!

I can't stress this enough. Some elites are just waaaayy too hard for the reward they give. Imagine coming face to face with an Invulnerable Minions - Horde - Illusionist - Jailer elite. Do you really think the reward you get from killing him will make up for the inevitably high repair bill? Trust me and avoid these beasts.

Gear

Most people assume that Inferno comes right after Hell, but in 90% of the cases it doesn't. You actually need to re run Act 3 and Act 4 hell until you have good enough gear for Inferno. Specifying which gear each class needs would make this post ultra long (not to mention that you can find this information everywhere), but at a minimum try to aim for a 500+ dps weapon and some Resist All gear.

Stay at The Earlier Acts

Each act of Inferno gets progressively harder, with monsters starting out at level 61 in Act 1, level 62 in Act2, and level 63 in Act 3 and Act 4. From my experience, Act 1 is a lot easier than Act 2 for the simple reason that Act 2 is full of ultra-fast monsters that pretty much insta pwn you. Because of this I recommend all players to farm Act 1 for a while until they feel geared enough for Act 2.

Enrage Timers

This is something all players should watch out for. Failing to kill Elites, Champions, and most Bosses in a certain time will cause them to enrage, making them impossible to kill in most cases.

Elites and Champions get an "Out of Time" buff when they enraged dealing massive damage over time to all nearby players. The timer for enrage seems to be 4 to 5 minutes.

Here is what the Bosses do when they enrage:

• The Butcher - Sets flame to all the floors.
• Ghom - Spawns gas clouds more frequently after 4 minutes.
• Zoltun Kulle - After 3 minutes will increase the frequency of his teleports and also dealing damage upon landing.
• Belial - If you keep Belial in his final phase for more than 3 minutes he will start dropping more green pools across the platform.
• Cydaea - Spawns spiderlings more frequently after 4 minutes.
• Rakanoth - Becomes much more aggressive after 3 minutes.

Bring a Tank

This one is for multiplayer only. Trust me, I have been in hundreds of parties and all of them fail without some sort of tank. When looking for games, make sure there is at least one Barbarian or Monk in the party. My best combination so far has been one tank + one ranged but I am sure other setups work too. 

Here is a video featuring Jay Wilson as he talks about Inferno shortly before the game's release:

If you want more information on Diablo 3 you might want to check out this Diablo 3 Inferno Codex review.

Peace!