The same as you remember everything else in the world: by repetition. Practice every day, write them by hand a hundred times like you did when you learned the letters in school, look at the mnemonics (funny associations with other shapes or objects) on Renshuu, YouTube etc., start reading simple words to practice them...
And don't hurry. Learning them "fast" is a sure recipe for getting them confused later and growing frustrated. These things take time and, as so many people on the Renshuu forums say, learning Japanese is a marathon, not a race. Getting acquainted with the hiragana and katakana, which will be your friends all along the way, is something you develop gradually, not something unpleasant you need to get rid of as quickly as possible.
I think it is worth learning them in the assigned phonetic rows (hiragana/katakana chart), as you'd need to be able to shift for verb tenses later. Learning them row‑by‑row helps your brain see patterns instead of 46 separate shapes. If you have blindspots with certain characters, find a real word containing that character (google/JISHO dictionary etc) and repeat the word.
It’s much easier to remember kana inside meaningful words than in isolation in my opinion. I am a newish to learning Japanese and forget stuff all the time. You will likely not be able see a new letter/word/kanji and recall it 10 mins later. You have to encounter it again and again!
