Turn It Off
If you want your iPhone to charge faster, anecdotal evidence suggests turning it off — so it's not using any power while it's juicing up — will cut down charge times.
If you don't want to hit the power button, then putting your handset in "Airplane Mode" (from the "Settings" menu) will stop your phone looking for cellular and Wi-Fi signals and can speed up the process.
At the very least, try not to use your phone while it's on charge. Lock the screen by hitting the sleep/wake button on the top right of your handset to give it a break while it recharges.
Use a Wall Charger
Keep It Cool
Did you know that a battery's ability to hold a charge is significantly degraded by extreme temperatures?
Apple states that "heat will degrade your battery's performance the most" and advises that you "keep your iPhone out of the sun or a hot car (including the glove box)."
Certain cases and covers that trap heat can also be problematic. If your iPhone gets hot while you charge it in a case, then you should notice a difference if you remove it prior to charging.
The official advice is to keep your iPhone as near room temperature (22 degrees Centigrade or 72 degrees Farenheit) as possible.