A while ago, my battery on my iPhone 3Gs wouldn't make it through a day and I was frusterated because I was going over my data plan for no apparent reason. In addition my phone was running painfully slow. Missed taps, slow home button response, and lots of lagging. Here are some steps I took to restore the speed, battery life, and reduce data usage.
Turn off Location services. I don't know why my dictionary app needs to know where I am when i'm using it... so turn off notification services for apps that don't need em'.
Turn Off Notifications. It seems like every app has the option to push notifications. Think if you really need your phone to buzz every time somebody posts on Facebook, or replies to your tweet.
As a side note, I've found a few studies that suggest when we use mobile phones and social media, we get stuck in dopamine loops. Emails, Tweets, Retweets, Facebook comments, Blog Views, etc. can all lead to dopamine releases in our brain (Aka: A Good Feeling). The bad part is, our brain than seeks more affirmation from Tweets, Retweets, Facebook comments, Blog Views, etc. The result is an "addiction" to surfing the web. I've found that breaking this cycle by not getting my phone to buzz every time I get an e-mail, FB message, Tweet, etc can really boost productivity, and keep me from wasting time on social media as much. I've also blocked Facebook from my laptop computer, which makes wasting time on Facebook even less of a problem.
Kent C. Berridge and Terry E. Robinson, What is the role of dopamine in reward: hedonic impact, reward learning, or incentive salience?: Brain Research Reviews, 28, 1998. 309–369.
Turn off Spotlight. I never use spotlight on my iphone (although maybe it might be usefull sometimes). Anyhow, If you turn this off for most/all the settings, the iphone has to work less hard to index all the crap on your phone.
Reset network settings. This delelets all the network information you have in your phone. It's a little bit of a pain because you have to go back and type in the wifi passwords for private networks that you might use, but it did seem to help make my phone run faster.