Thinking positive is huge! Another thing I did is set goals for myself and read them over everyday. Such as I will eat X amount of protein. The babies will weigh X. They will be vertex. They will be born after X wks gestation. I will drink X amount of water. Etc. I made them as statements of fact. Writing them down really helped me a lot. A twin pg can be very emotional and the hormones are outrageous. The written reminder can be a lifesaver sometimes.
You may be thinking that reading about some of the scarier side of a multiple birth will muddy the waters, and encourage you to think negatively or get anxious. The way I looked at it was that I wanted to be prepared and have at least a basic knowledge of anything that might happen. I think being knowledgable and prepared is a VERY positive thing. I think it will also inform as to what exactly the dangers are and how you might be able to prevent or at least prepare for them. I look at it as knowing as much as I can and preparing for what I can allows me to get a handle on the difficult things so I can focus my time and energy on the positive things.
So, in a nutshell, read about all that stuff, but once you have don't dwell on it, or make it your focus.
I hope you have a happy, healthy pg!