"…I always start opposite the valve, and work around from both sides to finish at the valve.
Just think about it for a moment; if you start at the valve, the bead of the tyre won't sit as low in the rim because the valve will be in the way; starting from opposite the valve, will make it easier to get the bead as low in the rim as possible, thus making it easier to get the bead over the rest of the rim. "
I dont usually post to these things but this tip helped me so much. theories aside.