OMG Kit that photo of that bike is too ludicrous! Looks incredibly uncomfortable, not to mention dorky! (hope I'm not insulting anyone) Looks like some should be riding a unicycle and pedaling it with their hands, standing on their head. lol!
As others have described, tuck in your tummy, tip your tailbone down so that your weight shifts onto sitbones instead of pubic bone. Some riders get lazy and let their back sag like a swayback horse and then put all their weight on their hands to hold them up. Get your weight distributed more back on your butt and your feet, it will strengthen your core.
Try to envision that you are 'walking' on your bike instead of rolling along like you are rolling dough with a rolling pin in your hands. That mental shift helps me when i catch myself getting lazy onto my hands.