Using dictionaries:

-My preferred method is to look the word up in a monolingual dictionary. If I feel it is necessary, I will then verify with wordreference.com. If I'm not feeling too lazy, I will include a context sentence with the flashcard. Also, with certain nouns I will list on the flash card relevant verbs (e.g. forthcoming).
-Also, I'll look up words I already know (recommended by Khatz). There are bound to be synonyms or other words in the definition that I don't know, so this is an easy way to accumulate more synonyms. E.g. if I look up "inundação," which I already knows means "flood," I find this:

1. Invasão de águas; grande cheia; alagamento.

 

I now look up "alagamento" (I know all the other words in the definition), but I can already guess that it means flood as well.