Never, ever:
1. Lease a car with an expectation of "gaming" the system and having equity to play with at the end of the lease, thus reducing your overall cost of ownership during the lease period. The leasing companies are smarter than you at setting the residual though yes, sometimes OEM will subsidize the residual value in an effort to reduce lease costs to consumers and sell more cars. But it's like the sports books in Vegas...those guys make a living on setting the right odds and over time you will lose if betting against them.
2. Make a significant down payment on a lease. Yes, it would lower your payment, but if your car is totaled the day after you drive it home, you would lose that downpayment. Your insurance company's "Gap" insurance will pay the leasing company but will not reimburse you for the thousands you put as a downpayment.
3. Lease if you expect to drive significantly more than 15,000 miles per year.
4. Lease if you like to keep your cars for more than 4-5 years (see #1 above).
5. Lease if you have substandard credit. The leasing company may approve your application, but it will add substantially to that low money factor you were quoted by the F&I manager at the dealer.
6. Assume that the money factor (interest rate) quoted by the F&I manager is the lowest one offered by the leasing company, even if you have perfect credit. They will often add percentage points to the rate if they think they can get away with it. That's pure profit to the dealer. Always ask to see the dealer's rate sheet from the leasing company. Edmunds.com and other sites have current lease money factors, residuals etc. for all makes/models. Do your research!
Leasing is good if:
1. you like to have new cars every 2-3 years.
2. you have good credit.
3. you drive less than 15,000 miles/year. Even better if, like me, you drive 7,500 miles because it drops the payment even more!
4. You like driving cars that are always under warranty.
When you compare annual cost of ownership, leasing is generally less expensive than buying a car IF those last 4 points apply to you. An extra benefit is that you don't take any residual risk...the leasing company does. If you buy a car and its value plunges in the used car market 4-6 years later then you will take all of that hit.
Good luck!