Hey - Been on Rebif myself for a long time now. I know I had the same question as you.
There are some good posts in here:post68440.html#p68440
I find it easiest to inject into my stomach, I've done 4 in a row all in my stomach, just moving a few inches each time, with no ill affects. I think as long as youre not seeing any injection site irritation you can do the same.
Hope this helps!