I have a good quality butchers block wooded chopping board. It is starting to curve due to being too wet at at times I suspect.

Laid on the worktop the left and right short edges are curving ever so slightly upwards.

Is there a trick to flatten the board again?

G T S, December 2014

The board will "cup" when the dryness of one side is different to the other, the wetter side swells across the grain. Try to even out the moisture.

Hammerite, January 2015

get it 100% dry and sand it level with a belt sander,but it will allways happen once your chopping wet food,

m, December 2014

You could try this method...
https://www.youtube.com/watch?v=FXuh_Q02DP8
But it creates lots of sawdust

You could soak it in water for a month and then press it flat for a year or so as it dries out.

You could let it dry out naturally and see if it mends itself.

Plastic chopping boards won't warp and they're inexpensive
Chopping mats even cheaper - Amazon sell them

Good luck..., December 2014