1. Always capitalize the first and the last word.
2. Capitalize all nouns, pronouns, adjectives, verbs, adverbs, and subordinate conjunctions (“as”, “because”, “although”).
3. Lowercase all articles, coordinate conjunctions (“and”, “or”, “nor”), and prepositions (under five characters).
4. Lowercase the “to” in an infinitive.

The U.S. Government Printing Office Style Manual offers one easy style:
“Capitalize all words in titles of publications and documents, except a, an, the, at, by, for, in, of, on, to, up, and, as, but, it, or, and nor.”
[from Writers.com]

Here are some sites for further exploration:
Writer’s Block
eHow
AdminSecret (cheat sheet)
Writers.com

Capitalize grandpa when it substitutes for the real name:
  Let’s go to Grandpa’s farm today.

Capitalize grandpa when followed by a name:
  Let’s go see Grandpa Bill.

Do not capitalize grandpa when it could refer to anyone:
  There are many grandpas at the retirement home.

Do not capitalize grandpa when preceded by a pronoun:
  I will go see my grandpa today.