Brushing portrait backgrounds effectively

[mattheww57828833]

Hi,

I have some portraits where I need to reduce the exposure on the background. I have been using a brush with the mask tool but I get this thin white line around the subjects. When I try to erase I then end up with a slight white halo. Is there any way either in Lightroom, or moving over to Photoshop that I can effectively eliminate this? Photo attached as example- see the hair of lady with glasses.

Any ideas appreciated! May be that there are some video tutorials that deal with this?

Many Thanks,

Matt