Frequent wear, timely oiling, getting wet and conditioning are the best ways to soften leather boots. It is a good idea to use multiple methods to soften boots, to speed up the process.
When oiling boots, use an oil that does not harm leather, such as mink oil. Gently rub the boots with this oil, but use a small amount to prevent darkening of the leather. To condition leather boots, use a leather conditioner, to help soften the leather and protect them. How often to condition leather boots, depends on the frequency of wear. Those people who wear their boots nearly every day can condition them about once a month.