I previously left my TN in the car on a hot day, and upon my return I found my leather started to dry up and my previous pocket stickers in my TN started to warp and peel off.
2. Don't use oily leather conditioners
Since my TN was dry, I thought putting some leather conditioner will restore the moisture. It did, BUT it made my TN a very dark shade of dark brown, almost black. It does, however, made it more resistant to scratches. Its a plus or minus, depending on the users preference.
3. Make sure it doesn't get wet
Normal water is fine. One time previously I got caught in the rain and got my TN wet, but once it dried off it returned to normal. What the TN cannot stand is beverages. I accidentally got some soy bean drops on the back of my passport TN, and the stain stayed till now.
4. Hold your TN more
The more you use, the more beautiful it becomes, the more resistant it is to wear. This is because it will absorb the small amounts of oils on your fingers, adding the protection barrier on the surface with a sufficient level.
5. If you want to clean and protect your TN:
Prevention is better than cure, so with my passport TN I've correctly used the right products for it. Amongst 2 products that I have found to be good on the TN are the Timberland boot sauce and balm proofer. Since timberland boots are suede (the most sensitive type of leather), these products can be used without discoloring the TN.
Use the boot sauce first and allow to dry. This will nourish the leather for many many months to come. Then apply the balm proofer after to give it a water repellent protection, which will not change the feel of the leather. Both are really good products. The best part is that it will allow you to still scratch your leather, giving it the rustic and used look.
Other leather conditioners are not specifically used for untreated leather, so be careful. Some will discolor your TN, while others will give your TN a glossy finish (since its made for waxed leather). The picture shows my regular Tn and passport TN, where the regular is very dark brown. Even though I use it more, but it does not sustain any visible scratches, unlike my passport TN.