In Page SEO Content Writing

Start with the low hanging fruit and then move on to content-driven links.

Low hanging:

• the resources Jason listed
• local listings
• local directories
• Program that encourages reviews from customers
• Reach out to partners to get listings on their sites as a recommended vendor (possible targets for a plumber might be contractors, real estate agents, home inspectors, lawn care companies, plumbing supply companies)

Content marketing/promotion is more challenging and requires more creativity, but also more defensible...don't think "how do I build links for a plumbing company." Instead, try to think of industries, people, topics, etc that are related to the space and then brainstorm to identify topics/content that they'd likely link to.  Some examples that might apply to this (these are just broad ideas to illustrate the point...no idea if the geographic market you're serving would generate enough incremental revenue to make the content development ideas worth the cost):

Related Topic: green energy

• potential target - green energy bloggers, plumbing suppliers, gardening bloggers
• potential content - comparison of different low-flow toilets, blog posts focused on lawn/plant watering tips (e.g.,"how to help save the earth without killing your lawn")

Related Industry: lawn care

• potential target - lawn care providers, contractors, sites focused on DIY sprinkler system installation, sprinkler system manufacturers
• potential content - page where you can enter in your lawn dimensions and it returns a suggested layout for a sprinkler system (distance between sprinklers, flow range of sprinkler heads, etc.).

Related Companies

• potential target - toilet manufacturers, sink manufacturers
• potential content - history of the industry, history of the technology, history of a specific vendor, why a new technology is better than the previous technology

Local

• potential target - local bloggers
• potential content - site that aggregates current watering restrictions for different municipalities in the region