A Good Memory is Essential For Vocabulary Improvement
In case you're wondering how memory can be so important in the process, think how hard it is to recall the phone number of your always-complaining-you-never-call-me grandmother, in comparison to how easy it is to remember your best friend's number (the person you call every day). Inevitably, repetition and frequent use have a role to play in memory and retention.
The same goes for vocabulary. If you want to expand and actually know your vocabulary at the unconscious level that is essential to fluent speech, you need to perform certain activities to keep your memory active and your vocabulary alive.

Repetition

Looking up a word's definition doesn't mean you've automatically added that word to your vocabulary (unfortunately). Unless you use the word repeatedly, you'll probably forget it fairly quickly. The most crucial yet systematically overlooked part of learning new vocabulary is your ability and willingness to put those words into use.
Frequent contextual usage will keep the word easily retrievable. The more you use it the faster you'll master it. Once you master a word, it's nearly impossible to forget it.

Curiosity

As kids, we tend to constantly bombard family and relatives with questions. However, this habit gradually fades as our own knowledge of the world reaches an efficient level that allows us to survive. Re-discover that natural propensity to be curious, and you'll discover new vocabulary wherever you look. For example, if you start looking for information on how airplanes fly, or how books are bound, you'll come across words you've never seen before, but will likely be able to use in the future.
By seeking information on subjects seemingly irrelevant to vocabulary, you won't be able to avoid a plethora of unknown words, which are just awaiting discovery. At the same time, you'll be working on expanding and keeping your memory fit, simply by revisiting past knowledge and building upon it further. This is one of the most effective, though indirect, ways of learning new vocabulary.

Exploration

Stepping out of your comfort zone immediately makes your senses sharper. Take advantage of this heightened awareness to build your vocabulary and stretch your memory.
Don't be afraid to explore unknown territories; pick up a novel in a genre you are not familiar with, or read a scientific journal article. Learning new things will improve your vocabulary knowledge and indirectly prompt your memory to absorb the incoming new data. The only prerequisite is willingness.

Practice

Memory is like any other skill; unless you work at it, it won't work for you. Fortunately, there are many activities that can help you sharpen your memory, like word games and quizzes. Such games and other interactive activities can be found in several vocabulary building software systems available on the market. Through word games, your memory will learn to be more effective in absorbing new information. In return, your enhanced skills will allow you to explore new vocabulary territories.