I've dealt with that situation by getting phone cases with an integrated battery that can augment the phone's battery. These cases take over when the phone battery starts running dry, eliminating any worry about getting through the day in my experience.
See related: iPhone 4S: Day 1 with an Android phone owner
My search for a battery case for the iPhone 4S turned up a lot of options, and I decided my main two criteria for a purchase was a case as thin/light as possible and one that offered roughly the same size battery as the iPhone 4S. I settled on the Incipio offGRID case as it meets my needs well.
The offGRID is a two piece plastic case that adds a little bulk to the thin iPhone while only weighing 1.9 ounces. That light weight adds a 1450 mAh battery to the mix, making it easy to get through even the busiest day while traveling and into the next with ease.
The iPhone 4S slips into the main case, plugging into the 30-pin iPhone connector. The small top of the case slides over the top, clipping firmly into place. The case provides cutouts for access to all phone controls, and a speaker on the bottom. The microUSB charging port is also on the bottom of the case, which is used with the USB cable (included) to charge both the case and the iPhone. Any standard power adapter that works with USB cables should also work just fine.
The thin strip below the iPhone on the front houses a tiny power button on the case and four LED indicators. The LED indicators function as a fuel gauge for the battery, and also indicate charging level when plugged in.
The battery in the case is manually operated, which I really like. Hold the power button on the case down for two seconds and the case battery takes over and both operates the phone and charges its battery. Hold the case power button down again for two seconds and the case is turned off, returning battery operation to the phone.
Using this method of operation means you can run the iPhone off its own battery until it gets low, then turn on the case battery. That adds hours of operation, and recharges the iPhone battery too. You could then run the iPhone for hours again even if the case battery is dry.
The retail price of the Incipio offGRID is about $70, but I found it online for less than $40. That is a great price for a quality battery case. I have always found Incipio products to be of good quality, and so far the offGRID is not disappointing me.