After you vacuum and lightly wipe your seat down, spray them liberally with a leather cleaner. After you spray them, use a brush to lightly agitate the cleaner into the surface. Then wipe clean. Repeat if necessary. Once clean, treat with leather conditioner. You should only have to do this twice a year. Use weekly vacuuming with a quick wipe down using a damp cloth (only water-no chemicals).
While there are brushes that are made and marketed for this, I simply use a boot brush that you can buy from Walmart (or wherever) for a couple of dollars. The bristles are light enough to not scratch the leather, but strong enough to agitate the dirt and bring it up. Do it one time and you'll never use anything else.
FocusFest 2013: Best of Show
Gateway FFW: Show Class Outstanding
September '13 ROTM