Learning vocabulary is probable themost boring task you face when you are learning a new language. That's why I like to know if you have any trick or mnemonic rule to do it.
I only know the hard way: learning by heart.
I disagree with the part about flashcards being a waste of time. For the last more than half a year I've consistently learnt about 300 spanish words per month (and yes, I remember them). But I build the flashcards dictionary myself (in an app, of course), so it's not only learning isolated words, but it's a way to help me learn words I need and often already have some context and use for (discussions, movies, clases, books, whatever). So yes, having a practical use for the vocabulary helps, but practicing it using flashcards helps too.
BTW, as for tricks or mnemonics, I think it's about simply using anything that works. Often it can be something rather silly (I remember "topo" as a mole looking out from the top of its hole) or "obvious" (I used to confuse "rezar" and "regar" until I noticed that Z kind of looks like somebody kneeling and praying). It can be even somewhat convoluted ("cabra" happens to be white like calcium is, and the translation is also used as a slang for woman breasts in my language, and so I remember it as two English words ca-bra; I have no idea how I managed to arrive to this mnemonic, but I'm so unlikely to ever undo it) or unlikely connection (I learnt "tornillo" by learning a salsa dance turn called that way). Really, anything that works, it's your head and it stays there (well, as long as you don't end up describing it on italki :) ).
Oh, and for me going over the flashcards is actually sort of fun. I try to do it as a relaxation after something, or when there's nothing better to do at the moment. That probably helps too. But yeah, as said above, this is just a support tool, probably nothing can beat remembering the words by putting them to an actual use.
I don't do any fancy stuff with the flashcards, I try to keep it as simple as possible. I think I have a good memory for written words, which may be a part of why this works so well for me, and for others pictures might make sense. So I use things like "ir" <-> "go", "caminar" <-> "walk" if possible, sometimes with prepositions if there's an obvious one to go with it. I use expressions or sentences only when I can't shorten it, so I have e.g. a card with "por lo que sé", I have a card like "Compra el vestido que tú prefieras" for this specific use of subjuntive, and I have "¿Quedamos a las 10 en la plaza?" for this very specific use of quedar, but generally I consider full sentences to contain unnecessary noise. As for nuances or synonyms, sometimes I can map a pair of foreign synonyms to a pair of my native words, and other times I do things like "arañar" <-> "scratch (animal)" for the context, but only up to a point, if I think my level is not yet sufficient for understanding the difference, I keep just one variant or even ignore the word completely for the time being. If I later advance enough to understand the difference, I add more variants or even redo and reset the cards if necessary. I see little point in getting everything perfectly right from the beginning, and I doubt it matters e.g. at A2 level to know the fine details of the zillion variants of expressing going.
But I think the main trick is that I do not do the flashcards as something isolated. I understand every card I create (as said above, if I don't, I drop it). And I usually have a context or use for it too, it may be a word I needed during today's class, or it may be those 30 words describing various parts of the body, or it may be a particular use of a word that I'd like to remember. So I already do have at least a very weak memory connection and flashcards just help to strengthen it.