Sure, I'll Call You Sometime
Only say this to someone you can be certain does not have your phone number. And only in a situation like a crowded event, where you can stage a quick disappearing act and "forget" to leave your business card.

I'm allergic
My favorite time to pull this excuse is when it doesn't quite make sense, but it's not outlandish enough to question it. Example:
"Want to grab a glass of wine or a drink sometime?"
"Actually, I'm Allergic"

What am I allergic to? Tannins? Alcohol? They'll never know. I've already staged a clever escape!

I promised my grandmother I'd help her with some chores
Nobody can ever get upset about someone assisting the elderly. Sometimes this is an actual valid excuse. Although I could very well blow off my sweet grandmother and she'd understand, most of the time I'd much rather hang out with her and talk about the ins and out of frying chicken.

Send me a proposal//resume/email/their information...
Immediately put the ball back in that person's court to do some legwork if they want you to do something for them. More often than not, they are too lazy to take the initiative. If they have to do some work, they more often think twice. It's a double filter. If their resume comes and it's crap, you can always send it back and tell them you'd be happy to look at it again once the changes are made. If they are persistent enough, then they might be worth some time!

People are crying
This means the people at this party are sappy and overly introspective and -emotional drunks, or they are catty fight-picking drunks. Neither are fun.

The booze is gone
Nuf said.

Your friend hit on someone's girlfriend and the someone wants revenge...
Grab your friend, no matter how amusing it is to watch the drunken smack talk and bail.

There are high schoolers there
This is never a good thing

They ask you to put your keys in a basket and draw out another pair...
Seriously?