1 - Simply ask the webmaster for a link! If your site can be a valuable resource for the other site's visitors, send the webmaster a polite, personalized email asking for a link. Tell the other webmaster why a link to your site would be beneficial to his visitors (and his site).
2 - Offer the other webmaster a freebie in exchange for a link. You could offer a free software download, a free service (such as search engine optimization for his site), or a testimonial for his site (with your link in it of course).
3 - Write articles about a topic in your field of expertise. Offer to let other webmasters use the article on their websites or in their newsletters. At the end of the article will be a "resource box" that provides information about you and a link to your web site. Writing free articles is a major traffic-builder!
4 - Produce and distribute free ebooks about topics in your field of expertise. Give (or better yet sell) other webmasters the right to distribute your ebook via their web sites or newsletters. Of course, each ebook will have information about your web site as well as a link to it!
5 - Exchange links with other web sites that compliment (but don't compete) with yours. Do a Google search on one of your target keywords. Contact each webmaster with a personalized email explaining why a link exchange would benefit both sites (be very specific). Then ask to exchange links. It helps to already have his "reciprocal" link in place before you contact him with the URL of the page on your site with his link on it.