While suicide bombers appear unstoppable and numerous, terrorist organizations actually have a difficult time recruiting them. This is especially the case when a lengthily campaign of suicide attacks does not appear to have accomplished anything. At that point, it no longer appears cool to be a suicide bomber. Since the most common recruits are young males, you have to maintain a successful public image in order to keep the potential recruits believing that sacrificing their lives for the cause is, well, worth the hassle. As has happened with all the major suicide bombing campaigns, eventually the good (smart and easy to handle) volunteers stop showing up. At that point, you have to use less desirable recruits, and techniques. You rarely hear of these, as the media like to play up how the suicide bombers are just like the "boy next door." But often that is not the case, and the story is rather more tragic.
For example, al Qaeda terrorists in Iraq have been studying the Tamil Tigers' (of Sri Lanka) practice of using handicapped people as suicide bombers. Apparently this makes "recruiting" of "volunteers" easier. All they have to do is find a paraplegic or legless person, strap him in the driver's seat of a vehicle loaded with explosives, and tie down the accelerator. In a related development, the drivers of some car-bombs are often not told that they're on a suicide mission. Reportedly some young foreign volunteers have been duped in this way. When they arrive in Iraq, they are asked to drive asked to carry some cargo from Point A to Point B by a particular route. The route takes them by a specific target, and the hidden explosives are detonated by remote control.
In Iraq, as in Israel, Pakistan and Sri Lanka, there has been increasing use of blackmail to force people to operate as suicide bombers. This technique has a broad range of victims to draw on. In all cases, having their dirty secret revealed to the community is considered a fate worse than death. For a woman, it can be adultery (punishable by death in many Moslem countries) or fornication (for unmarried women). For men, it can be something like getting caught passing information to the police, or owning too much money to a lender with nasty collection practices. If you are desperate, you can invent a case for someone being a police informer, and force the victim to "do the right thing" by his family and become a suicide bomber. Exploiting the mentally ill is sometimes done, but you have a more difficult time using someone who has a hard time following instructions.