I just passed my Norwegian language tests for immigration - got highest levels on oral, writing, reading and listening (yay me!) I've been living in Norway for 10 years now. The first two years I tried to get into the language courses at the university where I was working, but because I was on a year-by-year contract I wasn't high priority. I was number 180 on the waiting list year one and number 75 on the waiting list year two. I just learned on my own.

Here's my tips:

Watch movies in the language, with subtitles in the SAME language. Don't put English subtitles on your Spanish movie, put Spanish subtitles on your Spanish movie. You can read along and hear and it doesn't matter if you understand only 10% of the words. You'll pick up context and common phrases and even slang expressions. You get to see how the words are being pronounced. Watch one movie per week. Go back and rewatch your favorites, you'll find over time you understand more and more. Try to speak the lines along with the actors.

Learn songs in the language. Even better if you play guitar or piano and can play and sing along. Songs are some of the best way to memorize words. Run the lyrics of the songs through google translate and compare so you get the gist of what is being said. Try to learn one song a week.

As mentioned here, read children's books. Rhyming books, Doctor Seuss translations, Richard Scarry books, anything. Work your way up grade levels. If you can get ahold of 1st-2nd grade school books even better. Memorize children's rhymes.

Norway has a service called Klartale (clear speech), it's a simplified news service with easy words and very well pronounced Norwegian. You can read or listen or download podcasts. Maybe there is something similar in Spanish? Download podcasts or children's stories and play them in the car. Don't understand? Doesn't matter, try to repeat the sentences and phrases you hear. It may help to imagine you're an actor and try to ham up the accent as you visualize yourself as a Spanish movie star. It's partly about the mindset.

I bought the Pimsleur Norwegian courses. I think for Spanish they offer 90 lessons. It's (almost) entirely listen and repeat, which means you can run the lessons in the car or while walking to work. I had a half-hour walk to work which is the length of the lesson, so I did a lesson a day, once on the way to work and then repeating it on the way home. I recommend these. (I did 10 lessons in Spanish when I was going there for a conference and learned enough that I was able to ask someone on the street for directions to a pharmacy and understand enough words he told me to be able to find it.)

Extreme case: Immerse yourself in the language by moving to the country for 10 years. Be extremely frustrated for the first 6 years that you are living in a fog and not understanding shit. Be satisfied if after 10 years you can get by and don't care anymore about liberally mixing in English whenever you don't know a phrase. Tell yourself that the natives think it's charming how you mangle their language in amusing ways.

*Linguists -> Polyglots.

Duolingo is OK and offers a variety of languages, though take most of their reviews with a thumb-sized grain of salt. Their main strength is nudging you towards regular practice, and that's far more important than talent or motivation. Your resolve might waver, but you have a streak to uphold, so you'll log in and go through a few exercises. It's habit forming. Perhaps the most positive example of something being gamified. They're also great for forums and immersion training in some of the languages (Spanish, Portuguese, German and French IIRC).

My flatmate and his girlfriend use Italki, which they swear by, but I never tested myself.

Really, though, learn anywhere, but practice through immersion as fast and as often as you can. Read stories for kindergarteners and move forward. Watch something that interests you in that language. If you know some nature documentary/Godzilla movie by heart, look if it's available in the language you want to learn. If you're into "let's plays" or cartoons, look for those. You know the drill. Change the language in your OS/browser/whatever program you use most often once you'll feel you know it well enough to switch back if need be. Something I do to keep memorised words fresh is to say them aloud when they come up. Example: when I dress in the morning and pick up my trousers, I'll say "spodnie, pants/trousers, die Hose, los pantalones… etc." Try that for counting, getting change, groceries etc. Those are the words you're likely to need since they come up every day. There's time to know what 'encomiast' means, but it's far lower on the priority list than things you eat. Same goes for grammar. I know most of those weird "needn't have had who hadn't have needed" constructs, but day-to-day I rarely need more than conditionals and simple tenses.

Tried to learn German in Junior High School. Mostly failed, as evidenced during multiple trips to Germany.

Moved to Budapest when I was 30, and stayed for 7 years. Became pretty fluent, kinda because I had to. They don't speak a lot of English, and are were pretty hostile to foreigners. I learned through a combination of friends, girlfriends, landlords, and an immersive 1-week school in eastern Hungary where we spoke nothing but Hungarian all week.

I think several of the other commenters have touched on a key part of language learning that I think is missing... the musicality of the language.

First off, you need to be able to isolate one word from another, or even what sounds make up separate words. (This is ESPECIALLY hard in Hungarian, where the emphasis is always on the first syllable, and words can be 6-10 syllables long! "Holy shit! That was ALL ONE WORD?!?")

So the idea of watching a movie in Spanish, with Spanish subtitles on is a good idea. Helps you figure out the rhythm of the language, and where some letters are skipped or pronounced in an odd way. (There are 44 different letters in Hungarian!)

Which taps into the old wives' tale that musicians learn other languages more easily, because they have an ear for tones and beats. Mispronouncing a word is less of a problem if it is intonated in a way that is familiar to the native speaker. They can correct the weird "e" sound you made, because the rest of the 'music' of the word is correct and familiar.

So yeah... especially in Spanish (or most of the romance languages) you can get the conjugation or gender of a word phenomenally wrong, and they can still understand what you are saying.

Go bus I.

I bus go.

Bus go I.

All of those make sense (mostly) in Romance languages and English.

Just for fun, those sentences in Hungarian:

Megyek a buszon.

Busszal utazni.

Lovagalok a buszon.

En busszal fogok lovagolni.

Menj busszal.

... and other phrases, depending on many different factors.

(...and that's not including the 4 oddball diacriticals that I can't figure out how to create on my English keyboard)

Linguist & polyglot. Basically everything Devac said, but I wanted to add: listen to music in the target language! It's great for getting the sound of the language in your ears, which will help later with pronunciation (in the broad sense of capturing the "rhythm" and "feel" of a new language), and you get to discover cool new music! Sometimes I find this working the other way, as well--the thing that finally convinced me to start learning Spanish again was the desire to understand Jorge Drexler's lyrics.

Buena suerte, have fun, und bleib dran ;)