Steps to Speeding Up your iPhone 3G
1. Cleanup your browser cache frequently. Although the cache is to store files that are frequently accessed to speed up programs such as the Safari web browser, when it becomes too full it can actually hinder the performance of your device.
2. Whenever you use an application quit/ close the application properly. Because if a application isn't closed properly then the application will use some memory as well as the process to keep active
3. Install memory management apps in your iPhone. There are plenty of apps in istore.
4. Update iPhone. When people in the iPhone community complained of lags in the past, the issues causing them were fixed in the subsequent update. Make sure to keep your device up to date to make sure that everything is running as efficiently as possible.