In my opinion there are 3 things that drive customers and users to spread the word to others.

1. Product / Service Value - The perceived value (intangible) of the product or service into their daily life has improved some aspect of their day (saves time, reduces stress or gives them more freedom in accomplishing something). The actual value of the product or service is seen as a "good buy" based on the price and the mix of features, design and experience with the product or service.
2. Product / Service Quality - The perceived quality (intangible) speaks to the increased quality of life the customer believes has happened because of using the product or service. The actual quality of the product or service is based more on how well the product or service holds up (longevity) the materials used (weight, texture, design) and included guarantees (warranties, included extended service plans, etc).
3. Customer Service / Experience - This includes every interaction that the customer has when interacting with your venture / brand. This starts in research, includes online / in store shopping experience, purchase experience, return policy / experience and troubleshooting the product or service.

Keep in mind that each of the 3 points mentioned above include both their positive and negative states. A satisfied customer may tell (on average) 1 - 10 people (friends, family, coworkers, etc). However, a dissatisfied customer will tell potentially 10 times as many people about their bad experience (bad product, low quality product). Keep in mind that the internet and social media have greatly increased an individual customer's reach and has reduced the time it takes for opinions and thoughts to travel from person to person and multiple locations.

There are always different triggers based on the audience you're talking to. At its core though, there is one top-level notion that, if you focus on, lets you "plan for word of mouth"

Be authentic, Have a conversation

There is so much choice in just about every market now that you need to distinguish on more than product and service. Your customers want to feel valued and respected. They want everything to be perfect, but moreover they want to know that if anything does go wrong that it will be resolved, and your business will do anything it can to rectify or remedy the situation.

As soon as you make your business approachable, your customers, potential customers, and the people they talk to, will see you as more than a drain on their bank account.