Go for the low hanging fruit first. Identify businesses who have the worst websites / no on-site SEO / poor rankings and target them.
Ask them questions like 'would you be interested in hearing about how we can help you {generate more customers|increase your online sales|drive more traffic to your website|cut your advertising budget}?
Those are the type of questions that are going to get a 'yes' answer. And 'yes' answers lead nicely to the next stage, whether that's arranging a meeting, scheduling a call from you, or closing a sale.
Also, you could try saying something like 'after taking a look at your website, we've identified 20 important keywords your site isn't ranking for. These are all words people use when searching online for services you provide. And every one of them represents a customer who is potentially going to your competitors.... etc. etc.'
You'd want to word it in a way the customer will understand, but the point is to make them realise there is a lot of extra business they are missing out on - and you can do something about that for them.