Khushoo' is an Arabic word which corresponds to concentration and devotion in the prayer. It is a compulsory part of the prayer, and without it, the reward for the prayer may be significantly reduced, and may even reach the point where there is no reward for the prayer at all.

'Ammār ibn Yāsir narrated that the Prophet (may the peace and blessings of Allah be upon him) said: 

"A man returns after performing his prayer with a tenth of his prayer recorded for him, or a ninth, or an eighth, or a seventh, or a sixth, or a fifth, or a third, or a half."[Abū Dāwood: 796]

The essence of khushoo' is an awareness that you are standing before Allah. It begins in the heart, and is followed by stillness in the limbs. The following are a list of beneficial means of achieving khushoo' in the prayer:

1. Prepare yourself for prayer, by going to the bathroom, and performing wuḍoo properly and perfectly. The Prophet (may the peace and blessings of Allah be upon him) said: "No prayer should be performed when food has been served, nor should it be performed when a person is in need of relieving himself." [Muslim: 560]
2. Come to the prayer early, and wait for the prayer to begin. The Prophet (may the peace and blessings of Allah be upon him) said: "Everyone among you will be deemed to be occupied in prayer constantly so long as prayer detains him (from worldly concerns), and nothing prevents him from returning to his family but the prayer." [al-Bukhārī: 659]
3. Do not rush to the prayer, but walk slowly and calmly. The Prophet (may the peace and blessings of Allah be upon him said): "When the words of the iqāmah are pronounced, none of you should run to [the prayer], but walk with tranquillity and dignity; pray what you are in time for, and complete what has gone before." [al-Bukhārī: 636]
4. Prepare the place of prayer, so that you are comfortable, and not distracted. Aa'ishah narrated that: "The Prophet (may the peace and blessings of Allah be upon him) prayed on a garment with marks on it. During the prayer, he looked at its marks. So when he finished the prayer he said, 'Take this garment of mine to Abu Jahm and get me his inbijaniya [a woolen garment without marks], as it has diverted my attention from the prayer." [Bukhari: 373]
5. Make sure that you are praying with a sutrah to block others from disturbing you in the prayer. The Prophet (may the peace and blessings of Allah be upon him said: "When one of you places in front of him something such as the stick on the end of a saddle, he should pray and not worry about anyone who passes in front of that." [at-Tirmidhī: 335]
6. Pray your regular voluntary prayers before your obligatory prayer, as these will help you to get in the right frame of mind come the time of the obligatory prayer.
7. Put your worldly troubles on hold, and try to enjoy the prayer and find rest and tranquility in it. Don't see the prayer as a chore. The Prophet (may the peace and blessings of Allah be upon him) used to say: "O Bilal, perform the call to [begin] the prayer, and give us rest in it." [Abū Dāwood: 4985]
8. Remember who it is that you are standing before: Allah, the Lord of the Worlds. Aim for the best that you can get from your prayer, as the Prophet (may the peace and blessings of Allah be upon him) described the highest form of worship: "To worship Allah as if you see Him, and even though you cannot see him, He indeed sees you." [Muslim: 8]
9. Perform the prayer with tranquility in the movements. The Prophet (may the peace and blessings of Allah be upon him said: "The worst thief is one who steals in his prayer." The companions asked, "How can someone steal from his prayer?" The Prophet answered, "He does not complete his bowing and prostration with perfection." Or he said, "He does not make his back straight in bowing and prostration." [Ahmad: 22642]
10. Perform optional supplications and voluntary actions throughout the prayer. This helps to lengthen the prayer, particularly the parts of prayer that are normally shorter, like bowing and sitting.
11. Learn the meaning of what you say, so that you are not repeating large amounts of words that you do not understand. This is particularly important for Sūrah al-Fātiḥah, because the Prophet (may the peace and blessings of Allah be upon him) narrated that Allah said: "I have divided prayer between Myself and My servant into two halves, and My servant shall have what he has asked for. When the servant says: 'All praise belongs to Allah the Lord of the Worlds' Allah (mighty and majestic be He) says: 'My servant has praised Me'. And when he says: 'The Most Merciful, the Bestower of Mercy' Allah (mighty and majestic) says: My servant has extolled Me, and when he says: 'Master of the Day of Judgement', Allah (mighty and majestic) says: 'My servant has glorified Me' And when he says: 'You alone do we worship and from You alone do we seek help' He says: 'This is between Me and My servant, and My servant shall have what he has asked for.' And when he says: 'Guide us to the Straight Path, the path of those whom you have favoured, not of those upon whom is Your anger, nor of those who are astray.' He says: 'This is for My servant, and My servant shall have what he has asked for.'" [Muslim: 395]
12. Work to perfect your prayer, and to make it as near to the prayer of the Prophet (may the peace and blessings of Allah be upon him) as you can, because he said: "...and pray as you see me pray..." [al-Bukhārī: 631]
13. Perform the authentic prayers of remembrance at the end of the prayer, so that the prayer is completed with tranquility, just as it began. From among these is a supplication to make your prayer better, which you can find here: http://duas.com/dua/203/dua-after-the-last-tashahhud-and-before-salaam-5