It's possible for water to seep into leaves if the succulent is in direct sunlight. Make sure you get the whole leaf when using leaves. The rest of the pot should be taken with you. Fungusgnats prefer moist soil with lots decaying leaves and plant material. They'll be less likely to find a new home in a pot that's not kept clean. Soil plays an important part. Importantly, you must ensure that all soil has been thoroughly soaked. It shouldn't penetrate deeper than the top few inches of soil if you use very little water. If this seems too extensive, you can use well water, creek or pond water.