General Rules:

If a number is awkward or lengthy to write out (usually those numbers that require three or more words), then use the number symbols.

Examples:

use 155 not one hundred and fifty-five
use 1234 not one thousand two hundred and thirty-four

Write out any number that is two words or shorter:

use two not 2
use forty-one not 41

Note that the numbers one through nine, when coupled with multiples of ten (twenty, thirty, forty, and so on through ninety), use a hyphen to link to the multiple of ten:

twenty-two
thirty-five

Exceptions:

Feel free to replace even a one-word number with number symbols if numbers in a sequence are potentially confusing

hard to read: In my lifetime, I've seen five three-legged dogs.
easier: In my lifetime, I've seen 5 three-legged dogs.

Use number symbols for dates, addresses, times, statistics or mathematical information, exact money amounts, and other common numeric information.

dates: March 15, 1996

addresses:
10 Peyton Place
Washington, MA 01258

Tip: In a sentence, use a comma after the year or the state when more of a sentence follows.

times: 10:00 a.m.

statistics/ math. info.: 3/4 of all doctors recommend pills over liquids

money: $5.35

text references: page 2, chapter 9, paragraph 8, line 3

Note: The information on numbers follows MLA (Modern Language Association) and Diane Hacker's A Writer's Reference (2003, Fifth Edition) style. Other style manuals have other rules for numbers (e.g., APA- American Psychological Association).