Start with your interests
A huge mistake that language learners make is trying to remember words separately or out of context. The best thing to do, instead of thinking of separate words, such as ´come across´, is to start with the vocabulary that you are likely to have contact with. If you are interested in a particular activity or topic, it is much better to begin with this topic as you will use these words much more frequently. This will help with recall as the time between each usage will be smaller and you are much more likely to need these words in real life contexts.
Use the words in a context
The next point is something that I have touched on in the section about your interests. It is essential to use new vocabulary in a context or situation that you can later refer back to. This helps with recall and visualisation techniques for memory recall. If you use a certain word such as ´make a fool of yourself´ in a specific context, you will be able to remember where you were, what you were doing and who you were with. This is an essential part of reinforcing learned vocabulary.
Learn them as a short phrase
Many people don´t realize it, but learning a short phrase is much easier to remember that random words because these phrases have more meaning. If you want to recall the phrasal verb get around it is much easier to remember get around my city than the phrasal verb alone as it has more meaning. The same happens with bump into an old friend as you can relate it to people or specific situations.
Relate words to existing knowledge
I imagine that you have never really thought about it, but why do we translate new vocabulary? It is because we have the innate need to relate new information to things that we already know. The best way to increase vocabulary is to relate words to synonyms/antonyms/similar expressions. By doing this we can recall words such as be into as we know it is similar to like.
An element of relating words to information that we already know is grouping. If you sort and arrange words into groups you find them easier to recall. This is because you can link them to simple concepts that you already have reinforced in your long term memory.
Colour code vocabulary
If you colour code vocabulary based on specific verbs, prepositions or topics it can be easier to remember them, especially if you consider yourself a visual learner. I know that if I can associate a specific collocation with a specific preposition, I am much less likely to forget which preposition to use.
Games and challenges
It is very useful to test yourself on vocabulary and see what you can recall when put in a real life situation or under pressure. An element of competition is fun too. I remember that when I was at high school we studied in groups and we did recall activities, such as seeing how many countries in Europe we could remember in one minute. This forces us to recall less frequently used words more.
An important issue is the time between instances that we recall a word. For example, if it is a word that you see, hear or say every day, you will probably remember it easily. But if it is a word that you have only come into contact with once, you repeat it to yourself several times and then you later don´t use it for a year, you are far less likely to recall it easily.
A fun method of remembering vocabulary is with mnemonics. This is where we make use of elaborative encoding, retrieval cues, and imagery as specific tools to encode any given information in a way that allows for efficient storage and retrieval. I always use this as an example for native Spanish speakers. The pronunciation of the verb iron can be challenging buy if you think of it as /hay-n/, you can clearly recall how to pronounce it. A fun activity with images is to order the images and make up a story that links the images. This is very useful at the time or recalling and retrieving words to use them in context.