By carefully choosing your recommendations, you raise the degree of rapport between your website and its visitors. High rapport equals return traffic. Choose to link to the best resources you can find, even your competitors. Every website needs friends. Reciprocal linking is one of the most stable methods of promotion.
Links endure over time, regardless of positioning by search engines.
Seek out sites whose content complements yours. Link and benefit from a larger community to draw traffic from. Offer extra benefits if you like, but do so in an attitude of giving, rather than getting. Goodwill is paramount. A sincere, personal word of thanks is always appreciated.