I'm a waitress...so I'll help you guys out on this one.
It's our job to be friendly, pleasant and accomodating because the better we do that, the more money we get. Don't EVER attempt to gauge interest based on that. If you see your waitress look your way from time to time from a distance, DON'T read much into that either...we do that to look for signs that we need to check back...like your plate is empty or your drink needs to be refilled. A better way to guage interest is if she stays at your table longer than she has to and keeps the conversation going beyond where she could have ended it...but that's not even a sure thing.
Bottom line...if she finds you attractive, is available and knows you are available then you can get the most success by adjusting things in the following way:
1) Don't go to eat during the rush...she will be too busy.
2) Don't go in right before closing...she has side work and wants to get out of there...show up just in time to keep her from getting out of work and she won't like you, period.
3) Don't nurse your drink in a dining establishment for the whole night...you're costing her money. If you want to stay and drink, move to the bar...she'll still talk to you if she's interested.
4) Find a casual way to let her know you're available and interested...like asking "So where would I go to enjoy a conversation with you when I'm not hungry?" If she tells you where she hangs out, she's probably attracted to you and interested. If she doesn't, she's not.
5) And tip her really well. For those who aren't sure on how to tip...
10% is an insult. If you tip 10% you are telling her you thought her service sucked.
15% tells her you think her service was "ok".
20% tells her you feel you got good service.
25% or more tells her you think she's a great waitress.
If you under-tip her, she won't ever want to wait on you again...and she WILL remember you and consider you cheap.
If you tip her 25% or more she will remember you and want to wait on you anytime you come in.
Hope this helps...