First, let me say this: I HATE people who say they aren't language people or they don't have a knack for it. It's a shit ton of work no matter how you split it, but motivation and practice make perfect. If you can remember just one thing remember that: practice makes perfect, persistence wins the battle and attitude begets results. You can do it, even if it's after 1,000 failures.
If you think about it, you don't have an encyclopedic knowledge of every word and grammatical pattern in your native language (in this case, English) at your disposal. Also, shocking news: you probably only used about 10,000 words. And you probably use less than 200 grammatical patterns. And you're considered fluent. Congratulations. Okay, so how do we start adding languages?
1. It's not English, so don't force it into being English. A method might be to make an equation of a grammatical point. The boy kicks the ball SUBJECT(article NOUN) verb OBJECT(article NOUN). You can do that with Chinese, French, Greek, Nahuatl and so forth if you treat it that way. It might be old fashion and inaccurate, but it might help 80% of the time.
2. Try using your target language as often as you can - even if it's speaking to yourself. You will feel more confident with it and you'll get a feel for what is natural and what isn't. Open the refrigerator and think "I will open the fridge, I am opening the fridge, I opened the fridge" in your target language: you'll be using 2-3 grammatical patterns an elementary level, and you can build on it to higher levels. Speaking the language to yourself makes you comfortable with speaking in general.
3. If you have the opportunity to get a tutor (even if you're just talking to a friend who speaks the language), ask them if you can talk in the language and correct the most GLARING errors. By focusing on larger trends in your deficiencies, you'll break down your largest obstacles. As you go on, you'll save yourself from those mistakes, and then you won't make those mistakes anymore. And your tutor can correct the finer points as you progress. In the same vein, if you know that XYZ is "book," don't study it. "Study automatic vending machine" instead.
4. Don't stress over it. Don't expect perfection and don't worry about mistakes. You're obviously NOT XXXXX. Even if you were, you live in YYYYY. And unless you went to a language school or were immersed in language XXXX-ese since a young age, you're going to make mistakes. Most people you talk to will at least try to make the effort to communicate.
5. Also, while writing, reading, listening or speaking while thinking in your target language. It will cut down on time and you won't go "He said 'I will do to ihfiewhade... what is sugfuegw? OMFG I don't know this!!!! DEERINHEADLIGHTS'" If you're in language mode, you'll think "Okay he said "I will go to thejhdsgfjhbce because I'm hungry.' I thought I heard "student" in there... student... hungry.... ah-hah - student cafeteria.'" Just keep at it and it will become automatic.
6. Don't cram for 4 hours a day... at once. If you want to study 4 hours a day, do it in maybe 30 minute bursts a few hours apart. And it can be as simple as making a deck of 20 vocabulary flash cards here, reading from the text there, Skyping friend X on Thursdays for an hour. You'll digest the material easier and you'll become adept with switching gears quicker and quicker. Do you have an iPod touch or something like that? Download flash card apps - especially if you can make your own lists - to study on the go. Listening AND reading no matter where you go! You have your iPhone everywhere anyway. Whip it out do a few cards while you're waiting for the prof to come in, while you're in line at the store, poopin'. Seriously. We have some of the most ingeniously convenient technology and we carry it everywhere... and we use it for Plants vs Zombies. Download songs in your target language, watch TV and movies, read anything from shampoo labels to the newspaper. While we're at it, try writing a blog, diary or anything else.
7. You are an intelligent, bright person starting to speak a second (third, fourth...) language. Some people can speak very well in their second language, but only know a few words and fairly simple grammar (they stay at an intermediate level, you should be able to hold a basic debate or give broad-strokes explanation about something). You may not be able to deliver a speech in Japanese about the finer parts of thermonuclear dynamics, and no one expects you too, but surely you can tell some one about that funny thing that happened the other day. And which are you going to use most often?
8. Have fun with the words. Children learn languages by babbling. Try it. Say suihanki slowly, tasting each syllable. Say it in a yakuza accent. Say it fast, slow, angrily, lovingly -- sing it. Write the kanji and make up stories to remember them. Use your hands to act out a sentence. Engage your body and your mind.
9. Keep a daily journal in said language. Go back and review it every so often. Edit it, have people help you edit it. Use EVERY kanji you can as often as you can when reading or writing. Write vocab on index cards and stick them to things. "DOOR" goes on the door, WINDOW on the window... Seeing them daily helps make the language part of your daily life.
And visit this website: http://l2mastery.com/
Yeah. There are A MILLION ways to learn something.
Right now you're are 100% Engish. You're maybe 25% fluent in Japanese and 12% Chinese (for arguement's sake). You could have started the two at the same time, so long as you keep them separate there should be no problems. Say you have Chinese class 3 hours and 45 mins a week. Spend another 12 outside class and alternate it with 12 hours of Japanese a week (Monday Japanese, Tuesday Chinese....) a half hour here and a half hour there. Try and do it when you feel best. If you for some reason are a studying machine when watching prime time TV on Thursdays, that is 45 mins of, say writing kanji while doing flashcards during commercials. BOOM - done.
Don't forget to keep yourself interested and self motivated. You are not doing this for a just grade, right? You obviously want to learn the language(s). Why? You like the culture, you think it's a beautiful language, you want to live in China, Taiwan, Japan.... Whatever the reason, you have some sort of GOAL to obtain that's not just LEARN IT FOR FRIDAY'S TEST. You're learning it because it's something you WANT TO DO. Too many people say "When am I going to need to use French?" When you want to go to France, maybe? When you meet that special someone who's Swiss? When you go Quebec for business? In those three scenarios are three valid scenarios with different degrees of need behind them, all of which have some degree of motivation behind them. Find out your reason and never for get it. And never drop a language. It will come back to haunt you when you least expect it.
You're doing something very difficult and that is impressive in and of itself. If you're hazy but getting it, that's amazing. If you can make sense of it, you're pretty much a genius. If you have some rudimentary degree of command and proficiency when trying to make a complex sentence there is NOTHING stopping you from becoming fluent in time. It's not going to be over night. And it might not be in a year.It might take 10. But I'm willing to bet it'll be less than that.