The absolute best thing you can do for your kidneys, better than any supplement or scrip, is drink way more water than you think you need to. It's easy to check your electrolytes if you're getting a CMP on your regular bloodwork. Sodium is the most likely to fall out of range first, and if it's not regularly near the bottom of the range, you can definitely add more water to your daily routine.
And this has been said many times and I know you know it, but just in case some noob reads this comment, add cystatin to your bloodwork and calculate eGFR based on it, not creatinine. Cystatin is the most reliable metric for kidney function. Creatine can bork your creatinine based eGFR (and freak out your doctor!).