If you've got a singer in mind, be sure you attend one of their performances before you hire them. Even if you can't swing a free seat, you should be willing to pay for the performance. Small live music venues rarely cost more than $20 or so a ticket, which is much less than you'll be paying your performer.
Even if you don't have a date to impress or an event to book a performer for, you can still hire a singer -- for example, if you're a songwriter who can't sing. For recording purposes. it might be a good idea to form a partnership with a good singer who can handle your lyrics and writing style.