Multiple births are occasionally genetic and really just luck of the draw. Like the first response said, you will be so excited just to be pregnant, you won't care if it's one baby or 5. Especially for your first time, one will be a huge change to your life two or more can be extremely overwhelming.
Not that I know of... They either have to run in your family (fraternal twins...i.e. you naturally ovulate with more than one egg st a time & both get sperminated simultaneously), or nature just decided to split your one fertilized egg (identical). There are unnatural ways.. fertility drugs like Clomid or whatever... But I personally don't think taking drugs is worth it. You can try an herbal fertility tea: 1:1 ratios of dandelion root, rosehips, nettle leaf, red raspberry leaf, red clover leaf & vitex berries. It helps regulate for regular fertility, I don't know about multiples though.
I've never heard of any positions to ensure that you'll have twins. I can say I have one boy and he's such a joy. You would be happy with just one at first. They are amazing and wonderful. Hope u and your hubby the best.