Leaks depend on the ecig design, the amount to fill (or overfill), how much stuffing is in the cart, viscosity of the juice, and temperature.
When you fill the cart, suck up and liquid that pools on top using the bottle as a syringe. Turn the bottle right side up and squeeze it to create a vacuum, then suck up the excess.
The cart filler should fit snugly in the cart. If it's loose you will get leakage. If it's too tight, you will get dry and harsh vapor from atomizer starvation.
The more VG content, the thicker the liquid will be. Thin (all PG) liquids leak worse than thicker liquids.
Body heat can lead to leaks. If you put it in a pocket, find one that's looser. Mouthpiece down will allow leakage to go into the tip. Mouthpiece up will allow leakage into the atomizer area. You do have to clean the mouthpiece periodically with a rolled up paper towel (or remove the filler and wash out the plastic tip). You can remove the part with the hole in it and the secondary insert using a paper clip. Then you can clean down to the bottom.
Buy the Tank atomizer for your Ego. The tank leaks much less than carts. If you encounter a leaking tank, it's usually thin juice or a bad pierce when you insert it. Change tanks if this happens. Tanks like 70/30 down to 50/50 pg/vg. All PG will flood the atty, especially just before the tank runs out.