You should have fewer trips to the grocery store after reading and using the 101 Substitutions For Ingredients by author Christina Jones. Jones has compiled a neat selection of substitutions you can use when you discover you may be out of a needed ingredient. The one caveat to ingredient substitutions, at least what I've found in my 40+ years of cooking, is the substitution may not be exactly the same but it will suffice. In other words, use ingredient substitutes when you really need to.
Some of the ideas Jones offers in her book:
Allspice: Ingredient substitutes you can use if you are out of allspice.
Baking Powder: Ingredient substitutes you can use if you are out of baking powder.
Butter substitutes: Ingredient substitutes you can use if you are out of butter.
Her book also covers healthier cooking methods, snack substitutions, frosting substitutes and many other ideas that help make the task of cooking easier.
There is an interactive table of contents making navigation is easy.