XiaoBaiTu and other guru, pls Help on my parents' B2.

My parents got denied 4 times in the past, only my dad came here 10 years ago on business trip. I was converted from B1 to F1 and I guess that's the single big problem. I got green card. My parents both retired and have no child live in China now.

How should I do to increase the chance for them to get B2 visa in the April? I am thinking to invite them for my wedding in summer.
I remembered someone mentioned that asking senator write a letter may help. Any comments? how should I state the my request?

Also My parents have visited foreign cuntries such as: 2004 in Southeast Asia, and my mom alone visiting Australia in 2006.

Sorry for my lengthy question, but I am hoping they can show up in my wedding.


Thanks a million!