San Diego's casual lifestyle and year-round mild climate set the parameters for what to pack. You can leave formal clothes and cold-weather gear behind.
Plan on warm weather at any time of the year. Cottons, walking shorts, jeans, and T-shirts are the norm. Pack bathing suits and shorts regardless of the season. Few restaurants require a jacket and tie for men. Women may want to also bring something a little dressier than their sightseeing garb.
Evenings are cool, even in summer, so be sure to bring a sweater or a light jacket. Rainfall in San Diego isn't usually heavy; you won't need a raincoat except in winter, and even then, an umbrella may suffice.
Be sure you have comfortable walking shoes. Even if you don't walk much at home, you will probably find yourself covering miles while sightseeing on your vacation. Also bring a pair of sandals or water shoes for the beach.
Sunglasses and sunscreen are a must in San Diego. Binoculars can also come in handy, especially if you're in town during whale-watching season, from December through March.