How To Speed Up Your iPhone 3G / 3GS Running iOS 4

Been doing a bit of research on this topic; here is a synopsis of the tips I've collected from around the web, in no particular order...

1. Turn Off Spotlight Indexing

Navigate to Settings > General > Spotlight Search, and turn search indexing off for as many items as possible. "This reduces the overall workload of the system, so it appears to give an overall performance boost that is significant." (Source: RoughlyDrafted.com)

2. Cold Boot the iPhone

Press and hold the Sleep/Wake button along with the Home button for ten seconds or longer, which will kill the phone and initiate a reboot. You'll know you have achieved a cold boot when the screen goes completely dark and the Apple logo appears. Note that some users apparently have to hold the buttons down for longer; your mileage seemingly may vary. (Source: RoughlyDrafted.com)

3. Restore your iPhone using DFU mode.

Put your iPhone into DFU (Device Firmware Update) mode by following the instructions here or here. From there, restore your iPhone using iTunes. (Source: iSmashPhone / iPhoneHeat)

4. Reinstall your OS (and/or downgrade to 3.1.3) using iTunes.

Full instructions here, and here. (Credit: Findtut.com and Apple Insider)

EDIT: Lifehacker has an even more thorough article on this process.

5. Manually close your running apps.

Hit the Home button twice to bring up the list of running apps. Hold down one of the app icons in the system tray until all the icons start to wiggle, then hit the red minus signs to begin quitting the running apps one by one. (Source: poster Zaphon at Mac Rumors)

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Postscript: Here is an anecdotal report from at least one developer who seems to think that the current iOS 4.1 beta has improved the performance of his iPhone 3G. Hopefully, this bodes well...