What are some ways to build my list of mobile text subscribers?

You should begin by thinking of every customer (or fan) touch point that you have. Here are some places that you could put your YepWord and short code number (41242) so that people will see it and opt in: Emails, table displays, window displays, invoices, checks, bathroom stalls, high traffic areas in your store, website, business facebook fan page, business twitter page, press releases, print ads, tv ads, radio ads, social media ads, yellow page ads, youtube videos/ads, online articles, car wraps, billboard advertising, flyers, ect... Just remember that you need to give the customer a reason to opt in; maybe a coupon, or a discount.