I kept to a pretty limited palette for the background because although I felt it was important to include it as it helped create atmosphere, I didnt want all the detail in it to detract from my subjects and what they were doing.
My board has a tiny bit of a curve in it which doesn't bother me at all while painting and isn't usually noticable when I take a photo, but because this background is full of straight lines its a little noticable on the window sill. I promise you its straight really, although I suppose with an old building it could be slightly warped :)
1. Draw in the areas that you know you are going to mask just a touch darker than the rest.
2. Apply the MF thickly (I often apply 2 layers to get an even covering, and a thick enough covering to allow me to peel off the masking fluid in one piece once I have finished. This both preserves the fibres of the paper a little more and avoids you having to rub on the pencil marks causing them to get fainter or disappear altogether.
Hope this tip is useful :)