I'm a little late to the party, but here is my advice, from my own personal experience, understanding of neuroscience/psychology, and good hard research into the primary literature.
Step 1: Clearly and specifically list your communication goals. THIS IS THE MOST IMPORTANT STEP. A vague goal is: have a conversation in Spanish/"learn Spanish". A specific goal could be: In person, be able to ask for directions and understand spoken explanation/Over the telephone, order a take-out dinner from a restaurant/Watching a native drama, understand the basic plot of the episode/Read Harry Potter in Spanish in 1 month.
Goals tend to be focused on 1 or 2 of the 4 "skills" of a target language: Speaking, Listening, Reading, Writing.
Goals are specific about the skill level they want to achieve. For example, How quickly can you produce a novel sentence with that grammar pattern (short conversation pause/short stutter/slow and steady/quickly and proficiently)? Do you want to be able to enjoy cartoons/anime (usually spoken more slowly, simpler language targeted to younger viewers) or do you want to be able to watch the equivalent of Game of Thrones with archaic language in a weird accent?
Why is the first step the most important? If you don't know the
target skill or the
target proficiency, you're going to have difficulty choosing
how to practice and
when you've reached your goal.
If you don't know your goals yet, I would start some short term ones that you have a high confidence you'll achieve. The other posters have given good advice on the How to Practice side....but you need to be a discerning learner and figure out what the targets are first.
Step 2: Practice, practice, practice. Practice every day. Use an app like HabitRPG or Beeminder if you need motivation help. Language learning can be quick (see your goals!) but it tends to be a long term project that requires consistent effort over several years. The motivation component is important to your momentum, or at least, to not slide backwards (I am in this phase with one of my languages right now....and I'm ok with that!)
Free/cheap resources:
SRS Algorithms (explanation:
http://www.gwern.net/Spaced%20repetition) - Anki, Memrise, Duolingo, etc. etc. Use spaced repetition for vocab memorization, sentence formulae memorization, exposure to audio or audiovisual materials. I think Anki has one of the best SRS algorithms and communities out there, I've also used Duolingo a bit. I think Duolingo is a complementary thing but maybe not strong enough on it's own. It does try to get at all 4 skills which is good.
Lang-8 : writing and reading practice. Have your writing corrected by native speakers. Great for putting up assignments from a textbook (free corrections) or for daily diary entries.
RhinoSpike: Listening practice. Any text you have in the target language (eg, conversation in a workbook), put it up there, and a native speaker will record themselves reading it. Pros: Multiple speakers with slightly different accents, Male/Female, can request slow or fast reading speed. These features are important for language learning. Cons: Works best with short texts, as its free, and people don't want to spend as much time reading long things.
Skype: There are millions of people out there willing to be your language buddy. Make sure you clearly define which skype sessions are for their English, and which for your Spanish to avoid the 'tug of war' (
http://www.thepolyglotdream.com/how-language-exchange-is-like-tug-of-war-guest-post/) Other commenters have espoused Spanglish, but I would encourage you to set a very high bar for yourself for when you will 'cave' and use an English word. Circumlocution, asking your convo partner "you know that thing...?", dictionary lookup, etc.
Other peoples' good advice/more background:
http://en.wikipedia.org/wiki/Statistical_learning_in_language_acquisition (Adults also learn this way - the immersion technique isn't wrong! It's just, as some others have said, not always enough on it's own to reach goals)
http://en.wikipedia.org/wiki/Speech_perception (Focus mostly on the two parts about variation between male/female/child speakers and how phoneme inventories vary between languages)
http://confusedlaowai.com/2012/06/four-strands-language-learning/http://ilearnmandarin.blogspot.com/2013/01/developing-advanced-language-skills.htmlHope this doesn't turn into a tl:dr post :/ Oh well.