There are a lot of different skills that come into play in learning words. First, there is learning the word itself. That is, just placing the sounds of the word in your head. I can't tell you how many times a random word in a foreign language has just popped into my head suddenly, without any associated meaning. The word itself is just simple mellifluous cacophony superimposed on my conscious processes, floating without an anchor to any frame of reference.
Then there is the association with a meaning. Sometimes the memes associated with the words in each language don't mesh up on a one-to-one basis, and sometimes the meaning is slightly off from one language to the other, or there is really no exact correlate in another language. For example, the word "chciwy" means, literally translated, "want-y" (chęć is "to want"). But the closest translation is "greedy." Or the word "gwałtowny" means "rape-y" (gwałt means "rape"). But it is translated as violent, torrential and sudden as in a sudden storm.
So you have to work up the association with a meaning between the target language and the source language, and then do it also in the other direction. You just need to keep working it and working it in your head, and find as many tricks as you can to implant the associations in both directions.
One thing that is often recommended is to try to think of some sort of mnemonic or a word picture that helps you learn it, and the more silly, ridiculous or vulgar the mnemonic can be, the easier you will remember. For example, I learned the notes of the lines on the bass clef line many years ago with the mnemonic, "Good boys don't fuck animals." I dare you to get that out of your head. I read an interview with a memory champion once where he said that some of the images he used to remember items were just really, seriously wrong. He mentioned things like deviant acts that his grandmother was involved in.
OK, maybe that doesn't work for you. One trick I use a lot is to look at the patterns of vowels in each word and compare them in as many ways as I can. Look at the ones they have in common, look at how many vowels are in each word, think about their order, etc. If that doesn't stick, I'll do the same with the consonants, though that is usually harder. Sometimes I'll take a walk while I memorize words and try to recall what I was looking at when I thought about that word (sort of a soft version of the "method of loci", which I never really have much taken to in its pure form.
Verbal repetition works for me in some cases. Often if I take ten minutes to just repeat the English word and the Polish meaning over and over again, working it around in my head, I will know it pretty well.
Sometimes I will be thinking about a Polish word and all of a sudden a root part of the word that is similar to another word that has that root pops into my head as an epiphany. That almost always helps with learning the word.
It helps to approach the word as a thing of beauty or a piece of art to be appreciated, rather than an obstacle that needs to be overtaken. Polish is beautiful and intricate in the manner in which its structure is put together. Polish grammar is like high performance art mixed with some really fucked-up and arbitrary (pseudo) science. It works better for me to admire rather than memorize. The memorization will come, but it is better if it comes as an organic, natural process, instead of the pounding of pegs into holes.
Sometimes I encounter difficult words that have lots of lexemes (lots of disparate meanings). If this is the case, sometimes I will study them in a separate group, because they require a lot of mind-massaging to be remembered fully. And it is this class of words that reminds me the most that the most difficult part is creating the retrieval paths in the brain. That stuff all goes IN easily. It's in there somewhere, rattling around in the brain. It's finding it and being able to spit it out that requires the most effort. One proof of that is that when one is learning flashcards, one will learn the meanings of the word pretty easily at first, and be able to retrieve them for the first few days. But after that flash card has been retired for a while, and one comes back to it, one finds considerable difficulty, and has to recreate some of the pathway of retrieval. It's usually easy, then hard (but not as hard as originally), then easy, then hard (but less difficult)...it evens itself out, becoming less difficult in each iteration, until the word is learned, that is, until the retrieval occurs in a consistent fashion.
One visualization technique I use when I'm consistently not able to remember some of the lexemes of a word, is picture the neural connections between where the word is stored and the output of the lexeme that I am trying to retrieve from it. I picture the place in the brain where the word lies, the nerves with their dentrites and axons as conduit from the place of origination to the destination, and trying to get that lexeme from the dusty, dank place in the brain where is is stored, to the proper output vehicle (I picture the mouth, because one would be speaking the result), utilizing the fanciful neural path that I am imagining.
Maybe I'll revisit this topic later. I also want to talk later about "leeches" (words that are particularly difficult to memorize) and "push-outs" (words you really thought you had learned but have suddenly disappeared or become difficult). I want to talk about "practicing for diversion", which is a technique I use whereby I will work on learning some new words not necessarily to learn them, but as a diversionary tactic for my brain so I can go back to my original list after wiping my memory to see how well I really learned the words I was trying to learn.