Unfortunately, there is no simple tried and true way to reduce the duration of bleeding without hormonal intervention.
Anecdotally, we’ve heard of a few natural methods for reducing the days of bleeding. These include: drinking lemon juice at the beginning of the cycle, taking alfalfa or bioflavonoids, using a menstrual cup rather than pads or tampons, and even just being particular to eat breakfast every day and cut down on sugars and white flour. Sometimes, sitting and soaking in a bath can also help a woman get a hefsek taharah sooner. (Women with an IUD should check in with their physicians before using a menstrual cup.)
That being said, a woman eager to shorten the time spent in niddah should review the laws of the hefsek taharah and of staining. She should be sure to clean herself very well prior to making a hefsek taharah and to bring any questionable colors on the hefsek taharah or bedikot for halachic evaluation, rather than assume they are unacceptable.
Please write back with any further questions.