Is it really easy to watch everything we say? Have you ever said something that you wished you hadn't let out at all? I lost a good friend in this way and things were never the same. Like my friend always says, "Think once, twice then thrice before saying anything."
For me, the what's harder is when you are talking about someone (say your boss) and he or she appears. It is prudent to watch what we say because ultimately, it dictates the nature of our interactions.
This is not an easy thing for me to do. I don't have much of a filter and I am well-known for that. I'm a good guy, but people are always wondering what is going to be coming out of my mouth next.