Follow the server's subtle body language: If she lingers at the table, rearranging silverware and pouring you more water and wine, and looks you in the eyes, you're in the clear to chat. If she is looking around, has her body turned away from you, and doesn't respond with more than a short yes or no, it's time to cut the conversation short and wait until she comes around next time.
Never ask, "Sooo, what time you off?" Being overt to the point of rudeness will only earn you an obligatory eye-roll or a verbal put-down, not a date. Once, at a venerable Michelin-starred institution, a table of two young men wouldn't take "no" for an answer when inviting this former waitress to join them for a post-shift cocktail. When one joshed the other that where the waitress was really needed was in bed, she responded tartly with "Oh! Well, if that's the case, why don't I just give you my phone number and you can come over later and we'll have sex?" The astounded guys got it, meekly paid their bill, and left the restaurant.
Tip 20 percent. This is always the proper amount. Any less, you're a cheapskate; any more, you're desperate.
Buy whatever your server tells you to they should know the menu better than anyone, and by following their recommendations you can subtly show that you listen, and are interested in what they have to say.
Leave the restaurant before lingering too long. If you feel a connection, leave a note with your contact details. A direct ask-out is awkward: They say no, you feel like a jerk and have to leave the restaurant in shame. They say yes, they'll be mercilessly teased by eagle-eyed co-workers who've seen the whole thing go down, ruining the date before it ever takes place.
This is not always easy, but beats finding out your Match.com date lied about her weight by 20 pounds and has terrible taste in shoes.