PCB copper pour to prevent warping of pcb

What are the methods used to prevent warping of either double sided PCBs or single sided pcbs?

..eg, when the PCB is thin, and potentially warpable.

It is generally referred to as "copper thieving" and involves placing a pattern of repeated simple geometrical shapes or cross-hatched regions into large "void" or blank areas of a PCB. This practice is used not only to help balance thermal gradients of the board assembly when is is being fabricated, but also when it is being soldered. Additionally, keeping the layers to be etched with a higher average copper area tends to result in better, more accurate etching. It is especially troublesome if you have a sparse PCB circuit layer with either heavy copper or small (<6mil) track widths, and filling the void areas with simple patterns that won't create obnoxious bridges or potential layer shorts helps.

Balanced laminate.

you mean equal thickness say FR4 all the way across?

Equal copper on all opposing layers, so for a 4 layer board layer 1=4, layer 2=3

so single sided boards are likely to warp, I presume?

They may - or may not - warp to a degree which is of consquence to you - do you have any tolerance of warp/twist, coplanarity or mechanical fitment that you need to meet? What are the dimensions of your circuit board design? Type, size, and location of mounting holes? Very often with the fabricators I use, a two sided PCB cost is very, very minutely more than a 1 sided PCB, and the 2-sided board gives you so many more options, and improved solderability, it isn't worth it trying to pinch a penny or two on anything I work with

I was often told by my previous suppliers that it could be cheaper to make boards double sided because they bought in so much double sided laminate that the single sided was more expensive.

If you want to prevent them warping then as marce says - you need to ensure that the copper on each side is balanced no matter how many layers.
If you have a single sided board with copper all over one end and the other end free of it then it is likely to warp, balance it over the entire board and its less likely to warp however it can still warp because its not balanced both sides.
The thing to do is not put it through heat processes that will heat it to whatever temp makes it warp, when you get it from the supplier I assume that they put them under pressure when cooling down to prevent it?