I hate commenting on the internet, especially since it's going to go against almost every idea that the original commenter posted. That aside, here is my advice for studying Japanese. This is purely my opinion and one method that I've been using.

- Try to evaluate where you want to 'end up' with your fluency. If you want a full time job in a business environment then I highly recommend learning polite forms of verbs first. The more that your practice this form, the more comfortable it'll be to use. If you want to talk casually to friends and family, then learning just the plain form is ok. If you want the slow, long, methodical approach to learning Japanese and achieve a much greater fluency, learn the plain form and the negative form at the same time along with the polite form. I highly advise against using the plain form with people you've met for the first time or are dealing with in a business setting. It's very off-putting since it feels like you're talking like a child. I recommend learning the plain form and the negative form at the same time because of the structure of Japanese verbs. Learning both of these forms at the same time will give you an immediate hint as to how all of the other verb inflections should appear. This will become apparent when you're learning conditional, potential, volition and passive verbs.

- Tae Kim's guide is good for understanding many concepts quickly but the website I prefer the most is Imabi.net[1]. It contains more content and is constantly being updated and edited to improve its content.

- Learning katakana right after hiragana is vital if you want to live in Japan. Also, try to associate only the sound with the character, not a mnemonic. Use that time to focus on your pronunciation. Many borrowed words from other languages use katakana and learning katakana at the beginning will help you remember those words faster. For example, a common word used in Japanese is トレーニング. It sounds awfully close to "training" and since it's in katakana, it gives you a hint as to what the word might mean. While katakana words are not 100% borrowed words, it certainly appears to be above 80%. Related to this, do not learn romaji as a way of writing characters.

- I agree, do not avoid learning kanji. Always learn kanji with vocabulary. I don't recommend Heisig's book if you're taking the long, methodical approach I mentioned earlier. It includes a lot of mnemonics that I don't think actually contribute to the actual kanji's reading. Plus, if you do go through the entire series, then you'll still be behind in your kanji learning. There are a lot of words that you still don't know the meaning to! For example, 八百屋 and 海老 don't mean 800 shops and ocean's old. They mean produce market and shrimp respectively. My recommendation is to learn vocabulary and the kanji readings for that vocabulary.

- I don't read manga, personally, but another way to learn conversational Japanese is by watching television shows and mimicking them! (On the other hand, I don't recommend mimicking anime.) If you like manga, then go for it! Just go for your favourite entertainment medium. If you're learning conversational Japanese, then try to find a serious language partner. Find someone who is a native Japanese speaker trying to learn your native language. Allocate 1 hour or a half hour to go over Japanese and another equal block of time in your native language. Native English speakers will have the easiest time searching for a language partner but don't give up if your native language isn't English.

- Textbooks are good for certain circumstances. I believe textbooks are good for the long, slow methodical approach. It keeps you focused and gives you goals to work towards. If you don't like textbooks, then pick up a non-fiction book and try reading! It'll force you to pause, give thought to the meaning of the sentence and look up words/grammar. The non-fiction book method will probably take longer than a textbook though, especially if you're just starting out. Fiction books will be extremely difficult because they will sometimes make up words that won't appear in dictionaries.

- I actually prefer large lists of vocabulary. I get the greatest benefit from lists of nouns since I pick those up the fastest. As the original commenter pointed out, this is definitely not for everyone. Reading books or manga will give a more entertaining method for learning new words. It'll even give you the more nuanced meaning of words. The right and wrong places to use them, etc. Check out the Tatoeba[2] project as well for example sentences using the words you're learning.

For any language you're learning, try to use the language as you're learning it. Try to come up with new sentences based on what you've learned. Try to talk to people using the new verbs you learned. Try using Lang-8[3] for writing.

Japanese is really difficult because although I've been studying it for a year and a half now, I still don't feel like I'm 'fluent'. I can give you examples of things that I've been able to achieve using Japanese but I definitely wouldn't call myself fluent. Using the CEFR, I would probably put myself between B1 and B2 for certain circumstances and maybe between A2 and B1 for other circumstances. Learning Japanese will take time, don't give up.

[1] http://www.imabi.net/

[2] http://tatoeba.org/

[3] http://lang-8.com/