I have the same problem as the parent comment, and over time, I learned that people would take advantage of it, just as you mentioned. So, I decided to make my default response to every offer: 'Let me think about it, and I'll call you back.' Sometimes I only ask for one hour, but I always need some time to think on my own about the opportunity and make a sensible decision. This habit has improved things a lot for me!

Since getting married, I've gained an additional great excuse: 'I just need to check with my wife about this important decision.'