If you've had a kidney or urinary infection in the past year or so, if you have any sort of trouble urinating, if you have back pain or lower abdominal pain localized around the kidney area, if your urine is dark, or you have any other symptom of weak kidneys, and especially if you've been diagnosed with kidney stones, it would be wise to do a kidney flush first.
Otherwise, simply making a new life habit of drinking 1/2 your body weight in ounces of water every day (and starting a few days before liver flushing) should be enough. (A 150 pound person should drink 75 ounces of pure filtered or distilled water daily - that's water, not just fluids. If drinking caffeinated or sweetened drinks, you should drink more water.)
The Epsom Salts in many flushes are VERY dehydrating, so increasing your water intake before and during a flush helps tremendously.