What's the best way to keep your feet soft?

Moisturize. While there are many ways to soften the feet, i personally like applying moisturizer as regularly as possible particularly just before bed. You should see a noticeable difference by morning.
Moisturizer may work. If you have thick, dry and calloused skin on your feet, it may be a disease like psoriasis or ichtyosis. Certainly, prevent friction to your feet, and wear good shoes and socks, and use a moisturizing cream after your bath or shower every day, but if that is not helpful, you may try a cortisone cream, like lidex, (fluocinonide) by visiting a dermatologist.
Alpha hydroxy acids. Use a moisturizer that contains an Alpha hydroxy acid---lactic acid, glycolic acid. Ask your pharmacist for products that contain these ingredients.