With the exception of words ending in an -l (see the below paragraph), when appending a suffix, double the final consonant of the root word according to the following rules: 1) the word is monosyllabic and only one vowel precedes the final consonant (e.g., trip becomes tripped and tripping but head becomes headed and heading) or 2) the root word’s final syllable takes the main stress (e.g., occur becomes occurred and occurring but focus becomes focused and focusing). Exceptions, which must be expected from a natural language, exist—e.g., gallop (galloped, galloping), kidnap (kidnapped, kidnapping), program (programmed, programming).
When the root word ends in an -l, American English does not double the final consonant while British English does. Thus canceling, dialed, labeling, signaled, totaling, unraveled are American English and cancelling, dialled, labelling, signalled, totalling, unravelled are British English. (Some nouns, such as duel(l)er and jewel(l)er, follow this divide as well.) A notable exception in British English is parallel (paralleled and paralleling), which matches American English.
These rules may seem daunting, and their exceptions instill little confidence, but a careful editor—and most modern spell-checkers—should know well the correct forms of similar words. When in doubt, refer to a reliable dictionary.