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Bring an iPhone 3G back to life from iOS 4

So you've updated your iPhone 3G to iOS 4 and cannot use it anymore due to frequent freezes, seconds-long delays to get taps feedback, and the general sensation that "this is really sluggish!"?

Apple does not provide an official way to downgrade to a previous version of iOS. A real shame as iOS 4 is not usable at all on a 3G while 3.1.3 just worked like a charm.

The solution to downgrade is rather easy though. I won't make this a real tutorial given that there are already lots of them on the Internet, but here is what you have to do.

1. Grab a 3.1.3 firmware. I recommend looking around on this page.
2. Grab RecBoot, a utility that puts you in and out of the damn DFU/recovery mode.
3. Put your iPhone in DFU mode. Opt to restore, but alt+click the button in iTunes instead of just clicking it. Pick the 3.1.3 firmware that you just downloaded.
4. When the phone reboots you will get an error message. Nevermind, and use RecBoot to get out of the DFU mode.
5. Enjoy the activation process, and preferably, do start with a fresh setup rather than an old backup.
6. If you failed during this process then don't blame or sue me. Blame your brain and fingers.

In my case I recovered a fully functional iPhone 3G. I won't upgrade it again to iOS 4 unless Apple makes some serious performance improvements. I honestly think that they made a big big mistake here, making their happy customers angry ones.