The circuit proposed by Charles is useful in the case a 12V power supply is not available in your design. This circuit has a capacitor "always" charged and ready to add its voltage in series to the +5V power supply in order to get about 10V for a short lapse of time.
A "standard" 12V relay is supposed to get ON applying a voltage near 12V for less than 0.1 seconds. The voltage required to keep the relay ON is usually less than 5V.

However I do not like this circuit very much. It will not operate with many relays. Even if the relay gets ON, the maximum current it supports will be reduced considerably. Moreover, this circuit uses an additional 5V relay (RT contacts), which implies increasing the power supply demand.

In short: If a 12 V power supply is available in your circuit use the transistor option. If not, use a DC-DC converter; Maxim has some chips for that purpose. They are easy to use and cheap specially if your 12 V relay does not need more than about 30 mA.

Alfredo.