There are various techniques in the linguistic field dealing with vocabulary acquisition, even though some linguists do not think much of vocab acquisition.
The keyword strategy is when you correlate a difficult word in your L2 to a similar sounding word in your L1 as a means of memorizing the word. For example, when learning the French word mere ‘mother’, you could visualize your mother riding on a mare.
Some words are easier to learn than others based on word class. For example, nouns are easier to learn than verbs, likely because nouns tend to have a non-abstract, real world equivalent. Some words are easier or more difficult based on word appearance. Things like acoustic similarity to other words may confuse L2 learners. Word length and pronounceability may simply make a word more difficult to process, and thus more difficult to learn. For overlapping typologies, orthography may create a bridge for some words between the L1 and L2. The ways in which words are formed and their meaning may also play a role in learnability. For example, acronyms must be learned based on cultural knowledge, or homophones and metaphors may confuse readers who are not yet versed in these subjects.
You might also imitate the phonological loop and try to get natural repetition of words into the everyday studies of students. Repeat the words in realistic contexts. Come up with word banks, create realistic cloze tests alongside the wordbanks, etc. Students should also be encouraged to make notes about and look up difficult words (in a physical dictionary preferably).
Also, the linguist Tom Cobb created a sort of minimum "sufficiency" (referring to Krashen) necessary for L2 learners to acquire new vocab. The keys to this are increasing the knowledge of word families generally and having the specific vocabulary words reoccur (a minimum of six to nine encounters, but preferably more) throughout a class section.