Some suggested that you need to restart your iPhone preferably every day. This is partially correct.
Restarting your iPhone, or rebooting will clear cache stay in the memory, thus applications will run a little faster.
Like Safari, it is best to clear cache and history under Settings/Safari option.
But this does not help your iPhone back to the speed as it was when you first have it in hand.
The best way to make your iPhone like new, is to clear all unnecessary podcasts, videos, songs, SMSes, and call history.
You will notice respectable speed gain once you clear these regularly.
You may ask: How am I going to archive SMSes or call history since I need them as record.
Answer is simple, google iPhone SMS extractor, you may find tons of applications, either on Windows or Mac, that do the job with condition that you backup your iPhone regularly.
SImple, isn't it? My iPhone like new when I cleared all SMSes kept since 15 months ago, you may achieve the same by doing so.