If you are running iOS 4 on your iphone  3G and it was not as snappy as you expected to be, stop using it. iOS 4 is designated for iPhone 4 that has better processor and RAM, but if you persistently want this OS on your iPhone 3G/3GS these steps could be a way around it.

- Hard reset your iPhone 3G
By doing hard reset it clear out the busyness ever engaged off your phone, in this case do it twice. These are the steps to do it:
1.press and hold home button and sleep/wake button until the phone shuts off and restart.
2.Let go the buttons when the Apple silver logo appears. Now your phone had been hard reset.

- Disable the backgrounds
Backgrounds and shadow processing that give the icon looks better, sadly takes some amount of memory usage. It makes the iPhone rather slow. Disable it by following these steps:

1) SSH in as root and install ericas utilities
2) Go to System/Library/CoreServices/SpringBoard.app
3) Convert the plist we need from binary to xml so you can edit it in a text editor:
plutil -convert xml1 N82AP.plist
4) Edit N82AP.plist file, and change this line:
<key>homescreen-wallpaper</key>
<true/>

5) Save the file, respring (maybe reboot).
to this:
<key>homescreen-wallpaper</key>
<false/>

6) go to settings, general, spotlight and uncheck everything except apps and contacts.

- Restore the iPhone to Factory Settings
This method may sacrifice some of your not synchronized data, but some users said it really boost up the iPhone speed.

- Turn off Spotlight Search
Spotlight search give the workload to the system memory, to disable it just go to Settings -> General -> Home Button -> Spotlight Search, turn search indexing off for as many as you need. It will now decrease the work on the system.

- Downgrade your iOS 4 to 3.1.3
The iOS 4 it wasn’t meant to be on other than iPhone 4, by updating you 3G or 3GS to it, and then it will somehow slow your phone down. These methods disable most of iOS 4 great features, so there is no point upgrading your iPhone to the crippled iOS 4. Downgrading back to 3.1.3 might be the best choice.