Here's a list of current, up-to-date Spanish colloquialisms to spice up your vocabulary. I got them all out of Sin control; they're all used frequently by ordinary people. You might call them examples of respectable slang.
gorrear (v.)--to bum, to scam, to invite yourself to something. Me gorreó un cigarro--he bummed a cigarette off me.
a saco (adv.)--very hard, very fast, to the max. Trabajamos a saco--we worked our butts off.
ir / estar de juerga (v.)--to party, to go partying. Estábamos de juerga hasta las 6--we were partying till six.
ser pan comido (v. phr.)--to be easy, to be a snap. El exámen fue pan comido.--the exam was a snap.
un rollo (n.)--a drag, a bummer. La fiesta fue un rollo--the party was a drag.
tropecientos (mil) (n.)--a lot. Te lo he dicho tropecientas (mil) veces--I told you a thousand times.
fatal (adj. / adv.)--terrible. He dormido fatal--I slept badly.
genial (adj. / adv.)--wonderful. La película es genial--it's a great movie.
Feminine Language: I'm not being sexist, this is a concept in linguistics and philology. There are words and expressions that are used mostly or only by women; if men use them, it sounds juvenile or effeminate. Women who want to be perceived as no-nonsense or businesslike also avoid feminine language. Examples in American English are "precious", "sweet", and "cute". I can imagine some women saying "itty bitty", but not a man. This seems to be true in all languages. Here are some examples from Betty's Spanish.
chiflarle, molarle--function like gustarle and mean the same thing. ¡Me chiflan las fiestas!--I love the holidays!
mono--cute. Tu hermanito es muy mono--your little brother is really cute.
-ito--diminutive suffix. Me he comprado unos zapatitos y una faldita--I bought some (little) shoes and a (little) skirt.
-ísimo--augmentative suffix. Este niño es monísimo--this child is so cute.