What is the best mix for cement render?
I'd add a mortar plasticiser to a cement to building sand render mix made at a ratio of about 1:4 - I believe your namesake Travis Perkins offer a suitable product ;)

Damp the wall thoroughly before applying the render to slow down the transfer of water from the mortar to the wall.

I guess you could also use a layer of PVA ontop of the wall before rendering to provide some kind of barrier but that's not something I've done personally.