Find an English word that sounds like the Japanese word you want to remember. For example, the Japanese word "yowai" means weak and sounds like "Yo! Why?"
Make up a sentence to help you remember the word. In the above example, you might imagine a bully saying, "Yo! Why are you so weak?"
Repeat this practice for each word that you need to remember.
Think of a way in which the verb ending sounds like an English word or a Japanese word that you already know. The -masu ending sounds a bit like "missus," for instance, or you may be able to remember the -te form by thinking that the Japanese word for "hand" is "te."
Connect the word that you've thought of with the function of the verb form. The -masu form is a formal way of speaking Japanese, and addressing someone as "Missus" is also formal. To remember when to use the -masu form, you may think, "-masu, missus, formal." The -te form creates the present progressive, so you can think "-te, hand, What is the hand doing?"
Think of new mnemonics as you learn new verb forms.
Look at the kanji for the word that you want to remember.
Think of a picture that looks similar to the kanji.
Connect the picture in your head with the meaning of the word to help remember it. For example, the kanji for ooki, or "big," looks like a person standing with her arms and legs wide apart. You could think of that as the person making herself as big as possible, thus remembering that the kanji is for "ooki."