This is a female problem, and I'm not sure how you solve it. As a man, I keep my key fob in one pocket and my phone in another. The chance of me leaving both accidentally in the car at the same time and having my key in a place where it won't be recognized in the car are so remote, the chance can probably legally be considered zero. However, women often keep both the key and their phone in a purse, and for good reason. They frequently carry purses which pack ample storage space, and often don't have pants pockets.
I would recommend women pressure the clothing industry to start putting more pockets on pants. Pockets are quite useful. If I were a woman, I might would still keep the keys in a purse (because keys can occasionally be annoying in a pocket, and open you up to accidental button push risk) but the phone would go in a pants pocket.
Then again, most of you perhaps might take your phone out and dock it. Still harder to forget than hidden in a purse, but also not in your pocket. So pockets might not be as helpful to some. I always keep my phone in my pocket in the car, unless it's near dead and desperately needs a charge, which for me is a rare thing.
Historically, this wasn't as much a problem, because the key had to go into the ignition, and you had to do something with that key when you stop, greatly decreasing the risk of accidentally leaving it in the car. Now that you don't need to insert the key into the car, the risk is far greater. One of the negatives of keyless ignition (not to suggest the negatives outweigh the positives, but just saying it wasn't all positives)