here are a few things that could increase you chances
Grow or Gain Weight
A recent study published by the American College of Obstetrics and Gynecology correlates the rise in multiple birth rates with rising rates of obesity. Research found that mothers with a BMI (Body Mass Index) of 30 or higer were significantly more likely to have twins. Again, this statistic only holds true for fraternal (dizygotic) twins. The research also showed that women of above-average height were also more likely to have multiples.
Grow Up: Wait Until You're Older
Older mothers are more likely to conceive twins than their younger counterparts. It's thought that the body accelerates ovulation as the biological clock starts ticking faster. Seventeen percent of mothers over age 45 have multiples. Wait five more years and the odds rise to one in nine! However, the risks also increase; older mothers have a higher rate of miscarriage and are more likely to experience problems such as gestational diabetes during their pregnancy; in addition, their babies are at higher risk for chromosomal abnormalities.
Have More Twins
Once you have had a multiple pregnancy, you are significantly more likely to conceive -- and deliver -- twins again! Some estimates suggest that mothers of twins are four times more likely to have twins again than a woman who has never been pregnant, or who only had a singleton.
Diet: Yams & Dairy
No one is quite sure why, but the Yoruba tribe in West Africa has the highest rate of twinning in the world. A study concluded that the mother's diet was the cause, being high in cassava, a type of yam or sweet potato. The peelings of this vegetable are thought to contain a chemical that causes hyperovulation. In addition, a 2006 study found that women who consume dairy are five times as likely to have twins.
Seek Fertility Assistance
Reproductive technology has dramatically increased the multiple birth rate. Drugs that stimualate ovulation can lead to a multiple pregnancy, but multiples can also result from invitro fertilization. It's not just that multiple embryos are implanted in the mother, but there is also an unexplained increase in the number of monozygotic twins among IVF patients. No ethical doctor would provide treatment if it wasn't warranted, so fertility assistance should only be sought out when necessary
Have A Big Family
The more kids you have, the more likely you are to conceive twins in a subsequent pregnancy. No one knows the magic limit that triggers a multiple pregnancy, so you'll just have to keep trying until it happens.
Conceive While Breastfeeding
Most people think that you can't get pregnant while breastfeeding, that the process of lactating keeps a woman from ovulating. However, plenty of mothers of twins can disprove that theory. Some research has supported the theory that the chance of twins or multiples is increased if a woman conceives while breastfeeding.
Get Pregnant On the Pill
Birth control pills are usually thought to be 99.9 percent effective at preventing pregnancy. However, that .01 percent often results in a double whammy -- multiples. Sometimes pregnancy occurs when the pill isn't taken consistently; in other cases, the hormonal mix of a particular drug type simply doesn't provide enough coverage to completely prevent ovulation. In either case, playing around with hormones can lead to hyperovulation, increasing the chances of multiples.
Just Get Lucky!
Many parents of multiples don't meet any of the classic criteria, yet find themselves doubly blessed. Monozygotic twins are particularly mysterious; no one is exactly certain what causes an egg to split after conception, producing identical twins. The bottom line is that there truly isn't a whole lot an individual can do to influence their chances of having twins; sometimes you just get lucky!
you can read more about all there here
http://multiples.about.com/od/funfacts/tp/howtohavetwins.htm