Don't blink, this is so easy and simple you might miss it!
Most companies that specialize in hair have an easy game plan when it comes to making the leave in stuff so I'm going to let the cat out of the bag. Leave in hair conditioner, for the most part is nothing more than dilluted regular hair conditioner in a spray bottle. So to break it down for you, pour one part regular hair conditoner into your spraybottle and add 4 parts water then put the cap back on and shake, shake, shake it up and use.
For that crisis moment when your out of regular conditioner, fabric softener dilluted in equal parts with water and mixed well will leave your hair soft and managable.
If your just trying to keep the frizzies at bay you can rub a dryer sheet over your mane in a downward motion. Sounds crazy I know, but I have to say, it is a cheap fix!