What I am about to post here is valid for any 3G phone or device regardless of model but it is particularly focused towards the N series devices and their power hogging features.
Your battery life is dependant on many many things. How often you take calls on the device, the condition of your battery, the features you use on the device and so on and on. Therefore it is impossible to say that by following the information in this post you will get x amount of days battery life, but it will get you more time out of the battery than you otherwise would have got.
So with that out the way, if your looking to increase your battery life then follow these tips and your battery should start looking a lot healthier.
First of all lets start with THE big one. The one that is going to save you the most juice. Switching 3G off.
Yep, you heard me right. Just by switching the 3G capability of your phone off you will add hours and hours to your battery life. How is this so? Allow me to explain...
Due to the rather poor delivery of 3G in the UK by the network operators, it is rare for any 3G phone to maintain a constant 3G signal. Instead you will find that the phone constantly flips between 3G and GSM mode (Keep an eye on your signal one day). Even those of you on Vodafone who probably have the best 3G network coverage will find this is the case.
Unfortunately, this constant flipping between the two modes sucks power from the battery like a vampire as it alters its reception state for the different modes and the constant flipping is..well...causing it do this constantly! It can sometimes even make your phone unavailable for calls for very brief periods as it trips from GSM to 3G and vice versa.
If you need to use 3G for video calls or whatever then I'm afraid your just going to have to live with this but if you don't (And lets face it few of us do) then you can switch 3G off and increase your battery life considerably.
To do this, go into the "Settings" application (Found in the menu somewhere, by default Nokia normally stick it in "Tools"), and then to the "Phone" tab. In there you will see an option that says "Network mode" and you have a choice of "GSM" or "Dual Mode" (I.e. UMTS and GSM). Set it to GSM and your phone will restart. Once it restarts it will be working in GSM with GPRS speeds only but really for most purposes this is fine.
You have now just extended your battery capability considerably. You can further extend it by going to the "Connection" tab, going into "Packet data" and changing it to "When needed" so it is not constantly checking for a data connection.
The second big change you can make is to turn your phones wifi scanning capability off. The last time I looked not all Nokia's phones that have wifi capability can have their wifi cards switched off entirely but if you can, turn it off except for when you need to use it. Wifi is a power hog.
The next big change you can make is to lower the screen brightness settings on your phone. The less bright your screen is the less power is being used to light it up. Nokia by default leave the screen brightness at something like 50%. Lowering this a bit more will conserve more juice. Before you do this though please consider the fact that lowering the brightness setting will have a big impact on your ability to see the screen clearly in sunny conditions although you will be fine in the dark as you can't lower the brightness that far.
To lower the brightness, go to the settings tool in your phone and into the display option (Hidden in a subcategory called "Personalisation" on the N95). It won't hurt to set the power saving time out to 1 minute and the backlight time out to 10 seconds while your here (Although these are the Nokia default so they should already be set to this).
Finally in regards to the screen, although they may look pretty, animated screensavers use more battery power than the standard blank screen with time and date so avoid them if you can.
It also helps to keep Bluetooth switched off until you need it although the power savings are minimal in comparison to the other changes but every little milliamp counts!
Using the above methods I generally get about 3 to 4 days with about 3 hours talktime on my N95 without using Bluetooth, GPS or anything like that (I might be able to get more but so far I have not paid attention to the battery state before I put it on charge). If I am on a long train journey I can get about 4 hours worth of full screen video and about 2 hours talktime over the period of about 24 hours before it needs a recharge. As I said at the start of the post your mileage will vary greatly depending on how you use your device.
Hope this helps.