Of course you can’t keep your boots clean while on the trail, but once you are home take a damp rag and clean the dirt off the boots, especially at the joint line between the sole and the upper leather. A little running water and an old toothbrush might be necessary to clean around the sole edge. This area is especially important; it is where the dirt tends to grind into the leather, causing premature failure. Then treat the leather with your favorite leather conditioner. Nikwax, Sno-seal, Obenauf’s LP, beeswax compounds, or shoe grease are all OK. Don’t use paste shoe polish or neatsfoot oil, the former hardens and cracks the leather, the latter softens the leather too much. Work a little extra conditioner into the area between the upper and the sole.
Don’t worry too much about which conditioner you use. What is important is that you are comfortable with your favorite leather conditioner and that you use it regularly.
Keep your boots away from any sources of heat: campfires, heaters, car trunks, etc. will damage your boots.
A few other tips. After a day’s hike, take the arch supports out and allow the inside of the boots to dry. Don’t dry your boots by a source of high heat such as a campfire or a stove. A light coat of neutral (color) cream polish, Lexol or other light conditioner on the interior lining of the boots can help extend the life of the lining leather. Have the heels and soles repaired as soon as they become worn down. (I can provide that service if you do not have a local shoe repair shop). With a little care, your boots should last for years.