When someone asks you that question, what do you do? If you do not have a brief, clear, concise answer, you are going to miss out.
If it takes you more than a single sentence, you're sunk. You never want to answer something like, "Well, by day I work in an office, but on the side, I'm an independent distributor for a network marketing company". This response sounds like you are not committed and does not give the appropriate information. Plus, if you start off with "well", "uh" or any other word that creates a pause, you will not come across as confident and therefore have credibility issues.
Responses like, "I'm in the XYZ industry" don't help either. This tells the person nothing about what you actually do.
Make a plan for how you are going to answer this question. Next time try something like, "I'm in the health and wellness industry, which means we sell products that help with energy, stamina and general health". Or maybe something like this works better for you. "I'm in the communications industry, which means we can offer services to facilitate all your personal and business communication needs."
Note the term "which means". This is important because you want your prospect to have a clear understanding of what your business does.
What are you going to say the next time someone asks, "So, what do you do for a living?"