The ALLY Rule (Major)

This Rule is just a bit tricky, so read on carefully!
This Rule applies to those words in English with two or more syllables that end in

....ally

To translate them into Spanish, we will substitute the last two letters of the English ending with

...mente

Thus, the English word casually is the Spanish word casualmente.
However (here's the kicker), if the English word ends in ...cally (as in automatically) we keep the "c" and substitute ...ally with "amente" and the Spanish translation becomes automaticamente.
Here are some Examples

The ALLY Rule

English Spanish Helpful Reminders
...ally ...mente
...cally ...amente
Artificially Artificialmente
Anatomically Anatómicamente
Beneficially Beneficialmente
Bilaterally Bilateralmente
Casually Casualmente
Categorically Categóricamente
Classically Clasicalmente only 1 S
Decimally Decimalmente
Democratically Democráticamente
Economically Económicamente
Emotionally Emocionalmente C for T
Especially Especialmente
Facially Facialmente
Fanatically Fanáticamente
Genetically Genéticamente quirk #12
Generically Genéricamente quirk #12
Hermetically Herméticamente quirk #1
Heretically Heréticamente quirk #1
Hexagonally Hexagonalmente
Individually Individualmente
Initially Inicialmente C for T
Intrinsically Intrínsecamente
Ironically Irónicamente
Laterally Lateralmente
Legitimally Legítimamente
Logically Lógicamente
Mechanically Mecánicamente quirk #8
Medicinally Medicinalmente
Methodically Metódicamente no TH
Numerically Numéricamente
Neurologically Neurológicamente
Optically Ópticamente
Officially Oficialmente only 1 F
Partially Parcialmente
Peacefully Pacíficamente different spelling
Periodically Periodicalmente
Racially Racialmente
Radically Radicalmente
Rationally Racionalmente C for T
Sensually Sensualmente
Socially Socialmente
Specially Especialmente quirk #6
Specifically Específicamente quirk #6
Topically Tópicamente
Totally Totalmente
Umbilically Umbilicalmente
Universally Universalmente
Vaginally Vaginalmente
Virtually Virtualmente

You might take into account that there are many other words in English that end in ...ly. The above Rule takes into consideration only those that end in ...ally because there are very few exceptions to this Rule.

English words ending in ...ly (other than ....ally) can be translated to Spanish simply by preceding the ...mente ending with either an "a", or an "e", thus: ...mente or ...emente. But there are some exceptions (in other words, it doesn't always "work out").

English Spanish Helpful Reminders
Terribly Terriblemente
Candidly Cándidamente note the accent
Correctly Correctamente

The spelling of the word in the Spanish translation will sometimes vary a bit, but it will be close enough to be understood.
Here are some examples:

English Spanish Helpful Reminders
...bly ...mente
Appreciably Apreciablemente only 1 P
Calmly Calmadamente
Elegantly Elegantemente
Eloquently Elocuentemente C for Q
Fabulously Fabulosamente
Fervently Fervorosamente different spelling
Magnificently Magníficamente
Notably Notablemente
Necessarilly Necesariamente only 1 S
Oppulently Opulentemente
Patiently Pacientemente C for T
Perfectly Perfectamente
Variably Variablemente
Violently Violentamente

Whew! Well, that's as tough as it gets!
If you managed to get through this Rule unscathed, you might also try your hand at learning Venecian and Basque.
(And don't worry, you'll know the alloted number of words even without this Rule!).