So upon talking
To a few GW artists I was given a tip for freehand or what I call detail work this tip has helped me quite a bit with now only getting straight line when doing freehand but also my recipe for weathering I follow which is-(dirt (dried mud,btw some of the GW textured paints work wonders like agregellen earth,scratches,new blood old blood and the last ,chips)it helps when getting str8 lines for the scrathes.
The tip:use a light pressure when drawing so as to not let the body or resivoire of the brush bend,obviously the paint is thinned as well.and the second tip is - choke up on the brush as to where your fingertips are near the crimping (on the metal part basically where the hairs of the brush meet the metal.
And oh yeah PRACTICE.