Eating too fast or with your mouth open ( talking ) can cause gas as air gets mixed with your food.
Not chewing your food enough, especially with greens can cause gas.
Eating too much at one sitting can also cause gas.
Legumes ( beans ) and cruciferous vegetables ( cabbage family, broccoli, collard greens etc ) have a carbohydrate callled oligosaccharides which are hard to break down. Bacteria in your gut will finish the job, but a byproduct is gas.
I just wrote a post on how to deal with this with beanshttp://beforewisdom.com/blog/?p=1608
You can also buy "beanzyme" a natural enzyme to help you break down some of these oligosaccarides until your body gets used to eating a diet higher in roughage. If you ate very little fiber before going vegan, your system may not handle the higher fiber diet well for a timehttp://www.bean-zyme.com/Bean-zyme%20sa ... 20info.htm