Use an airplane-like landing motion with your brush when making contact with a surface near an edge. Land your brush about an inch in from the edge so you don’t drag it over the edge and cause runs. Land the brush softly while moving it in the direction away from the edge. Then brush across the surface and off the opposite edge.
Brush back and forth this way, landing an inch in from each edge and off the opposite edge until you’re ready to load the brush with more finish and begin your next brush strokes.