With the Marquee Tool selected, you also expand your selection holding the shift key + click and drag. That would add up to your existing selection.
You can also subtract, holding the alt key + click and drag.
There's also the option of clicking inside the selection and dragging, to change its position. That was already mentioned though.
You can feather your selection, by right-clicking on the selection and select "Feather...". You should experiment with this, since your image doesn't have hard edges and if you'd like to be a little more precise.
Hope that was helpful for future projects :) cheers!