What's a good way to attract customers when the most effective marketing strategy is plain word of mouth?

There is no simple answer to that.
Each market has its own hurdles and own benefits. Market reasearch is the key to understand the market.
Customers on a whole gets attracted to better products , quality products , unique and product that is giving a solution, ofcourse affordability cannot be ruled out.
Even after getting best of the best product cannot sustaint he market if availability of the product is low or customer support is not there.
Better service along with improving your product can help you to cater market in a better way.

Word of mouth works well in your existing circle but it's not a strategy for scaling a business since you have no control over someone referring you. Identify your target audience and develop a strategy to reach them.

Nobody Care's about products until they know that you care about them as a prospect.

Generate unique and memorable product packaging and branding.

1. Take written feedback from all customers
2. Create a plan for promoting this feedback across social media profiles
3. Create case studies that can be shared with potential customers