There are two ways of learning words: or by repetition or by inventing once any helpful trick.

Although every student has other preferences, I would say that  in the first case the best way is by a Spaced Repetition System, already described in other comments. Always try to add to the word a sample sentence. Memrise.com might be a great help if you don't want to create them on your own. For example B1 level in German is about 4000 Word and it will be easier to handle them digitally than with paper flash cards (in my opinion)

For the second, try to invent anything that let you remember more easily. For example if you mix up migi and hidari (one is left in Japanese, the other right) then remember the sentence "I would like to have the rights of Migi Mouse" and you will never forget that migi is right. Try to find anything similar in other languages, things that come to your mind when you hear the word. Imagine Shakespeare shaking a bear (or a beer). No help is too stupid. It is the opposite. The more stupid the rule, the easier it is to memorize.