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Stand, rather than kneeling. Shorten the distance between yourself and the subject (not always practical). Stop down; I think that the suggestion was f5 or 5.6 will give you better results than a wider aperture. Shoot later or earlier in the day; as the angle of the sun becomes more acute, the effect diminishes.
Interestingly, I gave the new 400 f2.8 II a workout on a brilliant sunny day on field turf where the heat waves were visible - everyone's nightmare. Shot from five yards behind the goal posts, sitting directly on the turf. My original 400 and 600 would have given me frame after frame of oof shots; in these conditions, I usually go with a 300 and chase the action along the sidelines until later in the afternoon when the sun is at a lower angle. The new lens had some heat haze issues at 100-80 yards away, but only intermitently and then it rapidly cleared up as the subject got closer, a benefit I never expected.