One key to memorization success is to attempt a manageable workload. This means that you want to choose a small group of words to learn at one time. One to five words is a manageable load that will enable you to be successful in remembering them for the long term. If you have longer lists of words to memorize, break the list down into chunks that you can learn each day.
One way to remember words and their definitions is to remember an association that you have with that word. The association may be an image, a person you know, a song with that word in it -- whatever helps you remember the word and what it means. You can make an association web by writing the word in the middle of a piece of paper and mapping out everything you associate with that word, or just by drawing a picture of the word and referring to it later.
Context is a large part of understanding and remembering definitions. While you may attempt to contextualize a word for yourself by making associations, it can also be useful to write the word in a sentence. If the word has multiple definitions, write it in several different sentences, each one creating a different context. If you are learning many words at a time, writing a story with all of your vocabulary words can be a fun and entertaining strategy for retaining knowledge. The more you use new words in context, the more likely you are to store them in your long-term memory.
Flashcards are a tried and true method of aiding memory. Rather than buying pre-made vocabulary flashcards, make your own with index cards. It will save you money and aid your memory by giving you one more chance to use your new words. Plus when you make your own flashcards you can personalize them according to your needs by drawing a picture on one side or writing the word in a sentence to help yourself remember the context. Quiz yourself or have a friend quiz you on the words.
When you learn a new word, one of the best strategies is to use the word in context immediately. Pick at least one new word a day to use in your everyday conversations. You might be able to fit your new words into emails and other correspondence as well. The more you use the new words, the better you will know them, until that knowledge is stored in your long-term memory forever.