honestly i say just go and stare at plants.. animals.. draw them as humans maybe, usually gets the brain going trying to transfer their certain characteristics into human form. if not, concepts as humans perhaps, red yellow and blue as humans? a certain fashion or architecture or idea as human? if u cant tell im in favor of drawing humans haha.. and maybe listening to music OUTSIDE of your comfort zone may get things flowing. if you are big into maybe rock you could switch over to jazz? psychedelia? avant-garde? very creative music mediums, usually aids in thought process.. if all else fails, force urself to draw until an idea hits you lol. or perhaps draw what you see, draw some still life for a while..