Sounds like a saliency detection algorithm would be appropriate as a pre-processing step prior to edge detection or thresholding. The choice of algorithm may depend on the image characteristics; eg, [1] utilizes a global center-surround, works well for large and distinct foregrounds, while [2] uses a FFT, works better for noisier backgrounds. There is a lot of literature on saliency detection, segmentation, etc.

[1] http://infoscience.epfl.ch/record/135217/files/1708.pdf

[2] http://www.klab.caltech.edu/~xhou/papers/cvpr07.pdf