The first thing to remember is that leather is skin, and needs to be treated accordingly. Here's my first rule: if I don't want it on my hands, I won't use it on a saddle. This includes things like Murphy's Oil Soap, bleach, ammonia, WD-40, olive oil, motor oil, Neatsfoot oil, petroleum jelly and Horseman's One-Step (has petroleum derivatives in it). No harsh products, no heavy oils, and nothing that isn't formulated for use on leather.
Second rule: Keep it clean! This doesn't mean you need to do a complete tear-down of your tack after every ride, but you should wipe it off after every use. A sponge or cloth dampened with water is all you need. Regular wiping-down gets the sweat off immediately and keeps dirt from building up.
If you aren't fanatical about wiping down, you're going to need to use something to remove the accumulated sweat and dirt. Again, plain water works well, but if you need a little help, Effax Leather Combi is my favorite - it cleans without stripping the moisture from the leather. Leather Therapy cleaner is another good product. Glycerine soap is acceptable, but should be wiped off completely. Don't use abrasives on leather, and if you're using metal polish on your bit or irons, remove them from your bridle and saddle first. And if you live in an area with lots of rain and high humidity, consider using a cleaner with either phenol (Leather Therapy) or tea tree oil to prevent the growth of mold/mildew (but remember that tea tree oil can be drying, so if you're using a product that contains tea tree, pay attention to the next part!).
Third rule: Condition properly! This means applying a light coat to ALL the leather you can reach, not just the seat and the outside of the flaps. Do the panels, the sweat flaps, the buckle guard, the underside of the flaps and jockey. About the only thing you can give a miss would be the billets - some people recommend that you never condition them (or your stirrup leathers), but I give mine a light coat a few times a year just to prevent cracking. Don't over-condition, and remember that oil really isn't necessary on today's English saddles, thanks to changes in the tanning processes. Over-conditioning will weaken the fibers in the leather, making it floppy and raising safety concerns.
Fourth rule: Be conscientious about your saddle. Don't leave it within reach of teeth, paws, claws or hooves. Store it in a cover on a rack in a cool, dry (preferably climate-controlled) area. If it gets wet, allow it to dry slowly and completely before you clean or condition it. If it's wool flocked, have the flocking and fit adjusted as necessary (usually every 6-12 months). If it's foam flocked, have the fit checked, too.
Now, here's an idea of what you can expect if you do - and don't - take care of your saddle.
First, a couple examples of well-cared-for used saddles. They show marks from the leathers and have definitely seen plenty of use, but they've been taken care of, and it shows.
Now for a mixed bag. The saddles on the right and left have received good care, but check out the one in the center - the finish has been completely rubbed off the flap, probably due to a combination of rough and or bad quality leathers, and a lack of care.
Now for the flip side of the coin. In this photo, the finish on the leather is gone, baby, gone:In this photo, the finish is compromised, and the stitching is coming loose:
Scary-looking scratches. Someone didn't follow the fourth rule:
This is why you never, ever ride in jeans (breeches have no inside leg seam for a reason):
Here's a good reason to pay attention to your billets - and to use a girth with a roller buckle! Though it's a little hard to see, there's a really deep split just above the 4th hole from the top on the left-hand billet.
Taking good care of your tack will really pay off in the long run. Conscientious care will not only protect your investment and extend its life, it will keep your tack in good repair - which could literally save your life.