This "prayer for guidance" is often used to help in important decision-making
Anytime a Muslim is making a decision, he or she should seek Allah's guidance and wisdom. Allah alone knows what is best for us, and there may be good in what we perceive as bad, and bad in what we perceive as good.
If you are ambivalent or unsure about a decision you have to make, there is a specific prayer for guidance (Salat-l-Istikhara) that you can do to ask for Allah's help in making your decision. Should you marry this certain person? Should you attend this graduate school? Should you take this job offer or that one? Allah knows what is best for you, and if you are not sure about a choice that you have, seek His guidance.
The Prophet Muhammad said, "If one of you is concerned about some practical undertaking, or about making plans for a journey, he should perform two cycles (rak'atain) of voluntary prayer." Then he/she should say the following du'a:
Arabic:
Translation:Oh Allah! I seek Your guidance by virtue of Your knowledge, and I seek ability by virtue of Your power, and I ask You of Your great bounty. You have power; I have none. And You know; I know not. You are the Knower of hidden things. 
Oh Allah! If in Your knowledge, (this matter*) is good for my religion, my livelihood and my affairs, immediate and in the future, then ordain it for me, make it easy for me, and bless it for me. And if in Your knowledge, (this matter*) is bad for my religion, my livelihood and my affairs, immediate and in the future, then turn it away from me, and turn me away from it. And ordain for me the good wherever it may be, and make me content with it.
Transliteration:
Allahumma inni astakheeroka bi ilmik. Wa'astaq-diroka biqodratik. Wa'as'aloka min fadlikal-azeem. Fa'innaka taqdiru wala aqdir. Wata lamo wala-a lam. Wa'anta-allamul ghuyoob.Allahumma in kunta ta lamu anna (hathal-amra*) khayul-lee fi deenee wama ashi wa ajila amri wa'ajilah, faqdorho lee, wayassirho lee, thomma-barik lee fih. Wa'in konta ta lamo anna (hathal-amra*) sharrul-lee fi deenee. Wama ashi. Wa ajila amri. Wa'ajilaho. Fasrifho annee. Wasrifnee anh. Waqdur leyal-khayr haytho kan. Thomma ardini bih.
* When making the du'a, the actual matter or decision should be mentioned instead of the words "hathal-amra" ("this matter").
* After doing salat-l-istikhara, you may feel more inclined toward a decision one way or the other.
