Geometry: Make sure your emitting surfaces are as low-polygons as possible. Usually a single sided polygon will suffice.
Not enclosed: If possible, do not completely enclose your emitters inside a dielectric object. All light that is emitted through the dielectric object is considered caustic light and will take longer to render.
Not intersecting: Make sure your emitter is not intersecting other geometry. In most cases this will not be a problem but it may add to the render time and produce strange results in the lighting. Regular geometry can intersect other geometry.
Strength: Keep your emitters at real world values. If your render is turning out too dark, don´t raise the power to several thousands of watts as that will break the realism and can also introduce more noise in the render. Instead, you should control the exposure by using the camera exposure settings (f-Stop, Shutter speed, ISO).