About.com's astrology page, astrology.com and signcompatibility.org offer guides to which signs get along, and which will clash against each other. One traditional rule is that zodiac signs of the same element (fire, water, air, or earth) are naturally harmonious.
Fire signs, Aries, Leo, and Sagittarius, are creative and spontaneous. Water signs — Cancer, Scorpio, and Pisces — are sensitive and emotional. Air signs, Gemini, Libra, and Aquarius, are intellectual and strong communicators, and earth signs — Taurus, Virgo, and Capricorn are practical and dependable.
Just as air feeds and gives energy to fire, those born under fire and air signs often get along. The intelligence of air signs inspire fire signs, who crave constant stimulation, and the energy and excitement fire signs provide is attractive to air signs. Just as water helps bring life out of fertile earth, the nurturing tendencies of water signs help to bring out the best from those born under earth signs. Earth signs can provide the emotional and financial stability water signs crave, while water signs can help soften their earth sign partners' outlook. Sensitive water signs can be hurt by the directness and somewhat selfish aspects of fire signs, and earth signs sometimes can find air signs flighty.