I try to reach from different directions:

• upper left (using my left hand)
• upper right (using my right hand)
• lower left (using my left hand)
• and lower right (using my right hand).

For the regions I missed, I have a soapy mesh/loofah thing held by some kind of rope on both ends and with both hands, I use it to reach the unreachable areas:

• left hand upper and right hand lower, loofah going diagonally (like this: \)
• right hand upper and left hand lower, loofah going diagonally (like this: /)
• both hands lower, loofah going horizontal (like this: —)

I suppose that should be enough, but I only do that once a week. For the rest of the week, I just reach as far as I can and let the soapy water do what it can.