One thing I usually do is take along a market umbrella and stand to make sure I can have some semblance of shade. Over the couse of an afternoon, it means I'll have to move it a few times to keep the shade over the keyboard. (I don't mind haveing the shade on me, either.) In addition I'm made a simple hood for the display to enable me to read it in spite of the ambient light. Then the remaining problem I've had to deal with is not being able to see the red LED's above my various switches on my particular board (Ketron MS60). That's something I've just had to learn to live with.
Paul C.