In the corner of my “cloud” toolbox, I always carry a small water sprayer. Not just any water sprayer, but one made for gardening with thick unbreakable plastic, big buttons and complementary colours.
Clouds need water to exist and I use my water sprayer (alongside larger ones) to moisten the air at the location where a cloud is to form. Basically, I build a wall of water vapour for the cloud to bump into. Water particles stick to the smoke and make it heavier, keeping the cloud together for just long enough to be photographed. Depending on the location and temperature I need to constantly adjust the amount of water I use.
This sprayer has been with me for five years now and has seen more places than any other ordinary water sprayer: from the US, through Europe, to China and even the desert in Australia. I think he even looks a bit satisfied.