I like to use a stick, a pin and a loop of string to open the sheds. It's a huge time saver!
I've made a video tutorial on how to do this for narrow bands, but this technique also works on wider pieces, too.
Push that stick up to the top of the loom.
You'll be going over the strands that you went under in the first shed, and under the 'overs'.
Adjust the length of each section of the loop.
And, Voila! you now have a handy, dandy way of opening both sheds!
And, here's the video tutorial: