I've had my boots for 3 years now and they still look brand new!
What I do is after each ride I wipe them down with a barely damp cloth to get all the dirt and dust off.
I wash my boots with Sergio Grasso Riding Boot Leather Cleaner (which can be found on Dover Saddlery) probably twice a month. Sergio Grasso Riding Boot Leather Cleaner smells so good and doesn't fade the black, I swear by this stuff! I never use tack cleaner on my boots or any kind of household cleaner. Oh, and never let your wet boots dry over a heat source, always let them air dry. I also use boot cream and sometimes even polish to keep the shine of the boots. What you do here is to a clean boot apply with a soft rag and then buff with a boot polishing brush and use another clean rag until the boot shines.
For the zipper I use a soft toothbrush to remove dust and sometimes get the toothbrush wet so I can remove salt.
Hope this helps!