GPS is the best option for navigation, now that you can use an IPhone or similar device.  There are several options for mounting them onto your handlebars, but the easiest to get hold of is probably the Ram Mount system.  A better solution that is also waterproof is the Dahon Biologic Iphone mount.  One of the advantages of having a very popular phone is the ability to source accessories such as this.

You could use a dedicated GPS but most of these will give you no advantage over your phone, and will mean carrying one more device and its entire infrastructure.

The IPhone comes with Google maps, which is great when you are at home.  The problem comes when you are somewhere foreign where your telco is likely to take the opportunity to charge a massive amount of money for downloads.  You need a dedicated GPS program that uses maps that you have downloaded at home, so you are not reliant on downloading data as you go.

ITunes lists hundreds of GPS applications, but most are not designed to replace a dedicated GPS device.  The free or nearly free applications will not contain a full suite of maps.  My choice is the TomTom application which for around $100 contains the full functionality of the dedicated TomTom GPS navigation devices.  That is, the application comes with the full map library.

Your next problem relates to power consumption. GPS devices are hungry, and you will either have to use your phone sparingly or rig up an external battery and charger.  Fortunately, this is not difficult, but will take a bit of effort.  You can buy an external battery for an IPhone, but be very careful about capacity.  Most of the ones I have seen will not give you a day of use.  You cannot use a generic USB device, as the IPhone requires a certain voltage on its USB data lines to work, that is, you must use an IPhone specific charger.  Lastly, you need your external battery mounted in such a way so that it does not compromise the integrity of any waterproof jacket.

The point here is that the USB charge system uses 5V.  If you buy a car charger, it will be designed to output 5V, and accept any input from around 7V up to around 16V.

You can buy lithium batteries designed for model electric cars including a charger for around $30 on the internet.  These batteries put out around 8.4V fully charged, and a little less than 7V when discharged.  They are often listed as 7.4V.  It is a good idea not to completely discharge a lithium battery, so you want one that has a larger capacity than you will actually use.  Lithium ion polymer is a better technology than standard Lithium ion.  Lithium ion polymer is often sold as LiPo.  These batteries have far more capacity than the dedicated Iphone external batteries.

There are many sites offering what you need.  For example, check out  Ebay often has good buys.  Buy a dedicated charger from whoever sells you the battery, and make sure that the charger runs on both 110V and 240V.

Assuming you want around six hours of use on your bike, then you will need to use in total 3000mAhrs at 5V.  Now any device designed to run off a USB charger will not require more than 500mA, that is milliamps, or 0.5 Amps.  So if you use 0.5A for 6hrs; that works out at 3 Amp hours, or 3000mAhrs.  Remember that if you charge your phone or GPS at night, you have its own battery capacity as well.  Most of the external battery packs designed for an IPhone have about a quarter to a half of this capacity.

This is all starting to get a bit technical, but if there is interest I will add a page giving detail about the addition of an external battery.  As a starting point, look at using one of the batteries described above, connected through a disassembled IPhone car charger. That is my personal solution.

Suffice it to say, that you can quite economically set up a GPS system using your smart phone that will give you all the information you need for a long day of riding.