"Holding Deposit"

It seems you made a mistake of not including the starting date in a lease.  Without a starting date, the lease is really not valid. 

Next time have the applicant to sign a "holding deposit agreement", and he pays you one month rent to hold the property for one month.  And when he moves in, you sign the actual lease, convert the holding deposit to security deposit, take first month rent, and exchange keys. 

The problem with signing a lease early and security deposit is that if tenant never moves in, you have to return the security deposit because in most states, security deposit pays only for tenant's damages.  If tenant never moves in, there is no damage therefore you can't keep the deposit for lost rent. 

A holding deposit agreement would say if you don't move in, your deposit would pay for lost rent due to your breach.