Hi Sheepstache,
This is the approach I've used to get to a level where people aren't sure whether I come from Spain or not when I speak (as an Australian and never having lived in Spain though I have visited it a fair bit) though I am not perfect, and will not claim to be.
* After typing this I feel that you can replace Spanish with whatever language you feel like and it will still work, I have also used it to get to a bilingual level in French.Acquire Assimil Spanish with Ease
http://www.amazon.com/Spanish-Ease-Assimil-Method-Books/dp/2700501314/ref=sr_1_1?ie=UTF8&qid=1351158403&sr=8-1&keywords=assimil+spanish+with+ease - how you do so is up to you; library, bookshop, google with filetype:pdf whatever suits your morals. Make sure to get the audio. Then follow the instructions.
Pimsleur Spanish, and Michel Thomas Spanish - also quite useful.
http://delicast.com/radio - pick your country, find a talk show/news station and listen to the radio as much as possible. All day at work/evenings at home, constantly immersed.
http://lang-8.com - crowd sourced correction of your written work by native speakers. Use
habitforge.com to make a habit of writing 100 words a day; then increase.
Find a frequency list of the 5000 most frequent Spanish words (such as this one
http://en.wiktionary.org/wiki/Wiktionary:Frequency_lists#Spanish) and once you've got a grasp of basic grammar, brute force it. Flashcards/Anki -
http://ankisrs.net/ will help with this.
I also used
http://busuu.com for vocabulary. It's decent as a starting place.
I have an e-reader, and read constantly in Spanish, there are plenty available if you know where to look. You could do the same/or just hire books from the library. Start with Harry Potter/something equally simple to read which you know the story of already. Or the bible if you're so inclined. Whatever floats your boat and is something that you know reasonably well.
Once you're not completely shithouse at speaking; join your local
http://www.meetup.com/ Spanish meet-up (you're bound to have one in most major cities) or put an ad up for a Spanish exchange. Meet weekly, drink, practice, flirt, whatever as long as you're doing at least an hour of speaking a week.
Have a bit of a google for a website which explains grammar in a way you understand, I didn't need anything more than Michel Thomas and Assimil as I already spoke French. But there's a bunch of resources online for grammar.
http://wordreference.com will save you having to buy a dictionary and the forums are pretty good.
The
http://how-to-learn-any-language.com forum is also pretty good as a source of info/inspiration. I also had a log there for quite a while.
Ultimately, resources are never the problem. You need to find a reason that'll motivate you to push through the plateaus and the problems you'll eventually have with obtuse grammar points. Find a study-buddy/hot person of your desired sex/someone you admire for their extreme polyglottery and challenge yourselves to progress.
I hope this helps, your milage may vary, but if you don't fuck around and devote a good number of hours a day/week you should do well. If you've got any questions I am happy to answer.
T.