The key deciding factor in conceiving twins is family history, but there are some things you can do to boost your chances. Some research suggests that taking folic acid will help. Other studies cite increased dairy consumption and higher maternal weight as probable indicators of having twins. At least one study saw a huge jump in twin birth rate among mothers who were nursing at the time of conception, with a rate nine times higher than mothers who were not nursing. If none of those works for you, keep trying. Research shows that women with more children are more likely to conceive twins.
Although I don't think a mmethod has been invented specifically to make twins or other multiples yet, in vitro fertilization has been known to yield a much higher rate of multiples than the regular method of getting a sperm cell to an egg. Twins are also more like to be born if you have a family history of twins. The more, the better.