I have a few rules over buying stuff.
1) I will not buy anything over the phone. If asked to (and I haven't hung up in panic by this point) I will just say "I don't buy anything over the phone".
2) I will not sign up to charities on the street or on the doorstep. If they ask, I will say "I prefer to sign up to charities on the internet".
3) I will not buy anything if someone is pressurising me or making me uncomfortable. If needed, I will say "I'll come back later" and leave/move away. Most of the time, I don't go back
It's not failproof, but knowing what I'll say in certain circumstances means that I don't end up buying stuff or signing up for stuff I don't want. My standby for situations where I don't know what else to say is "I'm sorry, I'm not interested". I find it a lot easier when I'm with someone