There are many ways to improve battery life on your Dell Streak, but there are two in particular that have made a significant difference not just on my Streak but on every Android device I've ever used. They're a departure from the obvious "lower your screen brightness" and "turn off 3G when you don't need it" tips, so I want to share them with you now.
The first is to change the WiFi sleep policy. You would think the opposite is true, but letting WiFi go to sleep when the Streak's screen is off actually hits the battery harder than keeping it awake all the time. The reason is that when WiFi is sleeping, power-hungry 3G wakes up to retrieve email, sync info, and get other data.
To change it, go to Settings -> Wireless & networks -> Wi-Fi settings. Then tap Menu -> Advanced -> Wi-Fi sleep policy. Select "Never" from the menu.
The second tip is to disable WiFi network notifications. You may think it's useful for the Streak to let you know whenever there's an open network nearby, but that "help" is coming at the expense of battery life because the device is constantly scanning and looking for networks to tell you about.
To disable it, go to Settings -> Wireless & networks -> Wi-Fi settings and uncheck "Network notifications."
Again, there are other ways to increase the amount of time your Streak can spend away from an outlet, but these two have made the most difference for me.