In the last seven months, the U.S. Army has met or exceeded all of its recruiting goals. In that time, over 160,000 people have enlisted, or re-enlisted. The total strength of the active duty and reserve forces are 1.2 million men and women, all of them volunteers.
Except for a few months in 2004-5, the military has been able to maintain its strength, despite wartime conditions. The biggest problem has not been casualties (only about 10,000 soldiers have been killed or disabled so far, less than one percent of overall strength), but the disruption to family life caused by so many troops getting sent to combat zones. This discouraged re-enlistments in reserve units, although mainly among the non-combat troops. In combat units, re-enlistments were at record levels.
Interestingly, the biggest recruiting aid has been word-of-mouth from the troops themselves.