Part of mystery shopping is being a convincing actor. After all, you're pretending to be a regular shopper even though you're not. As a mystery shopper you often have to interact with employees while you're out on a job. This means they are going to ask you things about why you're shopping, what you're looking for and what you're doing today. Many customer service representatives and salespeople are good at not being too pushy with sales. This means they will often ask you about your life to build rapport. You can't tell them the whole truth, so sometimes it's best to come up with a persona or character that you can pretend to be.
Build your character around the shop you're on
This can be more difficult than it sounds. If you're doing a mystery shopping job at a ski and snowboard shop, but you don't ski, you have to at least make it seem like you're interested in skiing or snowboarding. A good place to start is by saying you don't know much about it but you'd like to learn more, or that you've always been interested in whatever you're shopping for. While this won't work for every assignment, it can help you get started. Never tell someone you're an expert, unless you really are, because they could call your bluff and blow your cover. Ignorance on a mystery shopping job isn't always a bad thing. In most cases you want to interact with the employee and citing ignorance and a reason to seek them out is a great way to start an interaction.
Do some minor research before you shop
When you are assigned a mystery shopping job, it's always a good idea to do some research. Know specifically where the store is, what it sells and what you'll need to know. While most of this will be provided by the client, it still doesn't hurt to do some research. Conducting research beforehand will help you know what your persona needs to deal with. A basic Google search is usually all you need.
Write your persona down and study it
Don't tell the employee conflicting stories of why you're there shopping. A good way to ensure you won't do this is to write out what the persona you will be acting out is. Once it is written down, you can study it for a few minutes before your job and you will most likely remember everything you need to. Some things to write down include: why you're shopping today, why you're looking at a specific item and what you need help with. Don't worry about creating a name or a whole identity, because in most cases you'll be paying with your credit or debit card, which has your real name on it.