Option 1. Check the hotel room for available ports or sockets to charge your phone, some hotels now offer a USB port alongside the standard plug. One overlooked place is the hotel TV, most newer televisions will have a USB port where you can charge your smartphone from. All you will need is your USB cable. Some hotel rooms also provide an iPod dock so you can charge your iPhone or iPod directly from there, but this is only good of course if you have an iPhone.
Option 2. Buy a USB plug for the desired county that your travelling to. These can be found easily on eBay, just type into the search bar the country you are travelling to, followed by 'USB plug' or click on one of the links below:
USA USB plugs on eBay
European USB plugs on eBay
United Kingdom USB plugs on eBay
Option 3. Ask at the front desk of the hotel to see if they have any chargers for your smartphone in their lost and found as guests often leave these behind. If you own a popular branded smartphone such as HTC, Blackberry, Samsung or Apple then it is more likely they will have a charger for you.
Option 4. Buy a plug adapter for the country your travelling to. This will mean you can put your home countries plug into the adapter and use it as normal, this is ideal if you want to use your existing smartphone charger but will mean less space if you want carry a few chargers for different devices. To save space it is better to charge all your devices from the USB port on your laptop as then you just need one plug along with a few cables which take up less space.