A few things that help words stick.
Read the foreign words aloud as you type or encounter them.
Having trouble remembering certain words? Give it a night. Sleep helps reinforce memory, and you'll be surprised at how easy you'll recall a word that felt impossible to remember the day before.
Don't stress about perfectly memorizing every word in a Word Group. If you've learned most of the words in a Word Group and feel ready to move on to the next Group, go for it.
Mentally picture the most difficult words to memorize. This can help reinforce spelling and recall.
In games that allow you to reveal the answer, try to give yourself a moment before revealing a word you can't remember. The effort required to recall a difficult word can go a long way in helping you memorize it.