Most of the times we give in to the temptation of saying yes to every opportunity or plan that comes our way, without even thinking about it deeply.
How many times have you said yes to a trip with your friends, only to find out 2 days prior to departure that you can't make it?
A lot, I guess. It sucks, doesn't it?
That's what happens when you say yes without thinking. It not only makes you look whimsical but also screws up other people's plans.
Save yourself some embarrassment and other people their time by taking some time to say yes to anything.
When somebody asks you:
"Hey, we're going for a road trip this weekend, wanna join?"
"Sure, I'd be happy to but let me check my calendar first and see that my schedule's clear for the weekend. Can I get back to you by tomorrow?"
Then take the time you've just bought yourself to think if you are willing to go on that trip and if you can actually make it there. Think of any existing plans that you might have for that time. Think of what's important to you.
Think before you say yes to anything. Always. Unless someone's offering you free pizza.