At the bar
Chatting someone up? Make yourself seem more valuable to pique their interest.
Look, when a person likes you, that’s it. You’ll know. But there’s really no harm in making yourself seem so in-demand that you can only meet at certain days or times... even if you’re really only eating pizza rolls in your sweat pants and watching Miami Vice with your cat for the rest of the week. It's called the scarcity principle and lots of pick up artist nerds love to use it as a cornerstone of their creepy ethos. That said, there's some truth to it -- just don't be a top-hatted jerk about it.
Trying to get in with a group at the bar? Identify the head honcho.
So proclaimed the daughters of House Spice “If you wanna be my lover, you gotta get with my friends.” You don’t have to be anyone’s lover, but if you want to make nice with a group of cool kids at the bar, you gotta get with the head cool kid. How? Identify the person everyone looks at when something funny happens, seeking reaction-based confirmation that yes, that '90s Will Ferrell reference was funny. That person is your target. Start a conversation, and if that person likes you, you're in the group.