[Chinahand]

The inherant problem with most DIY timber is that it has not been properly seasoned.

I would suggest that, before you fix anything to it, you place it flat on the floor of the room it will be used in. If it starts to curl up just turn it over and it should straighten itself out. Keep doing this until you get no further movement and then seal it all over with a good quality varnish or oil based paint.

 

[Chinahand]

With 19mm solid pine, once it has dried out and stabilised, no bracing will be necessary. In any case bracing would need to be at least 3" x 1" to be strong enough to counter any possible distortion of the pine shelf. It would also have to be dried and stabilised before use as otherwise it might, itself, cause the shelf to twist.

 

[Chinahand]

This is getting silly guys. Poor old 57XX must be wondering what sort of company he has fallen in with.