Good Battery, good grounds - especially at the rear fender connection, good starter (some of the old ones can get weak, full of carbon/dirt, etc.), multi grade oil (I use 5W-30 or 10W-30 synthetic blend), and a good tune.
I use Autolite 26 plugs in my M18 and KT-17II with good luck.
Another HUGE starting aid is a trickle charger like a "Battery Tender Jr." to keep the battery topped off and warm....especially after sitting a long time between snows.
My M18 fired up on about the 2nd rev. this morning in sub 20 deg. weather, and the KT-17II on about the 3rd rev. at 17 deg this afternoon.