Keep leather free of dust and debris by wiping boot down with a damp towel. This will keep the dust or debris from extracting essential oils from the leather and ensure the leather does not become dry or brittle, causing the leather to crack.
If chemicals make contact with your boots, make sure to wash off the chemicals as soon as possible to prevent chemicals from having adverse effects on leather.
Over time leather tends to dry out, which means the leather loses its oil content and you can see the leather fade and dull over time. When this happens, you need to get some oil back in the leather especially in the flex area of the boot. You can use any number of products to put oil back into the leather, including shoe polish, baby oil, Neat's Foot Oil, Mink Oil, Snow Seal, Silicone treatment, and there are probably several other similar products. Use a cotton ball to apply this material to the leather and focus your effort especially in the flex area of the boot at the sole line. This area close to the sole is the critical area that is most vulnerable. Keep this area oiled and the leather (and boot) will last a long time before it cracks and the boot is rendered "worn out".
Like all thing the more care you put into your boots the longer wear you will get from your boots.