I think the problem with flash cards is that you learn things in isolation. It's only natural that that would make them difficult to remember. You need to immediately APPLY whatever new thing you learn.
I am a musician and it's exactly the same with music (or anything else, for that matter). If I learn a certain chord form on the guitar neck but don't actually start USING it in my playing I will quickly forget it.
Regarding language, when you learn a new word, you also have to start USING it immediately, which means creating sentences with it. That way the word has a context for you, instead of just being some letters on a card.
I agree with the previous poster that reading is very good. Obviously all the words are in context when you read, and it's also a good idea to start reading stuff that you are actually interested in, because it will be more pleasurable than reading random stuff.
But then again, I also recommend you
think about which topics you would like to increase your vocabulary on and read specifically about those topics. For example, let's say you have noticed that you really lack vocabulary when it comes to the topic of kitchens and cooking. Then the obvious thing to do is start reading cookbooks/recipes in Spanish , and things like the kitchen area of department store websites in Spanish (like
http://www.elcorteingles.es).
A great advantage of department store websites is that they have photos of most of their products so that you also link an image to the word, which helps to remember it much better.
In fact, I highly recommend specific websites in general. It's a fantastic way to learn vocabulary for a specific area/topic.
Want to learn more words around the topic of train travel? Look up the Spanish national rail service website.
Want to know more words around the topic of banks? Look up websites of Spanish banks.
Want to know more words around the topic of supermarkets/food/shopping? Look up websites of Spanish supermarkets.
But really
think about what topics interest you and you want to learn more about, then simply go online and look for Spanish websites of companies, stores, government organizations, museums, institutions, schools etc. etc.