Tribal designs for body art are ever popular, and this applies to pectoral tattoos as well. Classic and neo tribal designs work well, as they can easily be extended into a tattoo sleeve at a later date, if desired. Other styles of tribal tattoo, such as Maori and Polynesian, can just as easily be developed into body art for your chest.
Japanese-style tattoos are another idea for a pectoral design. There are many different aspects to Japanese tattoos, so you could concentrate on just one in your design, or combine a few. Common features of Japanese body art include dragons, koi, foo dogs, waves, wind bars, cherry blossoms and maple leaves. If you use more than one of these features, the waves or wind bars are used to unify the design -- the waves for beings of the water, such as koi, and the wind bars for beings of the air, such as dragons.
Heart tattoos work well on the chest, since your heart is roughly below where your tattoo lies. Sacred hearts are a popular tattoo design and are used often in the old-school or traditional style of tattooing. The sacred heart is a catholic symbol, so some people may get this tattoo for the religious connotations. There are also many other heart designs you could go with. Ask your tattoo artist if you need help coming up with a specific design.
Although the term "pectoral" is most often used to describe the male chest, it is not uncommon for a women to get tattooed on her chest. If you'd like to go for a more specifically feminine design, there are several options. Some ideas include swallows, rose blooms or other flowers, butterflies, cherries or other fruits, anchors, cupcakes, and stars. Traditional-style crossed pistols also look great on the chest area.