Another method is to hand paint markings as mentioned, you need a steady hand, you can also make stencils by using a craft knife and masking tape, this is fine for basic crosses and stars etc. Not a big fan of decals as they can stick out like sore thumb as too neat on a nicely detailed tank. I have some pics somewhere of germans actually painting numbers on, they used stencils and brushes, will see if I can find it.