*cringes at thought of never cleaning leather tack* My gear gets a good clean at least once a week, without fail, including a wipe over to remove sweat after every use. My saddle is 15years old and minus a few wear marks from the stirrups, looks brand new.
I use Effax leather conditioner, with a good glycerine saddle soap for cleaning really grimy leather.
Depending on how bad your gear is, you may need to use a knife to scrape built in dirt and dried sweat away. This stuff will ruin a good leather saddle faster than anything else.
Once you've removed the bulk of the solid grime, I use a soft, lightly textured damp cloth and work the saddle soap right in, using small circular motions with my hand. This lifts the dirt off effectively and will also remove sweat in the leather.
Once soaped, follow with a good quality leather conditioner. As I said above, I chose to use Effax products as they do not leave a slimey residue behind. Work the conditioner into the leather, and allow to soak for a day, then come back with a soft cloth to polish.
If your leather is cracked or really stiff, you'll need to use a good quality leather oil. Don't use it on the seat of your saddle, or the girth points. The rough side of he leather is the side that will absorb the oil, so apply oil here generously. I then put my leather into a dark plastic rubbish bag and leave somewhere warm for the day. This gives the oil the best chance to soak right into the leather.
You just don't want to oil too frequently, as the leather can start to swell and stretch.
Make sure you pull your bridle apart too - no being lazy and just cleaning around the buckles, as these are the most important parts. If your leather starts cracking and stiffening at these points, your setting yourself up for a snapped bridle, probably while you're galloping or jumping, or at a show!!!!