You can't say you have a solid grasp on a foreign language unless you have a solid vocabulary to back your claims up. Grammar, spelling and pronunciation mistakes might slip every once in a while, but if you're reading a text or listening to a conversation and can't understand a third of the words used there, what's the point? There are several techniques that are used in building up your vocabulary in a foreign language and we'll try to cover some of the most common, which you can practice without spending too much or looking too far for them.

Reading new texts and their translation

A lot of language learning books rely on this technique to get the student's vocabulary up and running from the first lessons. The students are given a rough text in the foreign language (the difficulty level of the test is dependant on the level the student is considered to having reached so far) containing some new, more complex words with each sentence. Below the text, an exact translation is given, with contextual annotations if it's the case. After the translation, the words that are being considered as "new" are usually given a definition of and if they are used in a specific context in the above mentioned text, this is also mentioned.

Picking words out of the context

Oftentimes, you'll be reading a text or listening to someone speak in a foreign language that you know and spot a word that you don't understand. Focusing on the context in which it is told however, will often give you a clue about what it means. If this doesn't happen and you're totally in the dark about what that particular word means (oftentimes convoluting the entire context too) you can look it up in a dictionary.

Visiting the native speakers

Nothing will boost your vocabulary faster and better than a visit to a native speaking country of the particular language you want to expand in. Even a one-week trip will teach you more new words than an entire course in that language, not to mention spending a few months out there with the natives. This obviously perfects your grammar, spelling and pronunciation of words together with your vocabulary, which can only be beneficial. The only problem might be the costs such a trip would take and the amount of free time you will need for it.

This said, feel free to skim the Internet for any sources that can help you expand your vocabulary in a particular foreign language. After all, you can't simply take the dictionary and start learning new words by heart; you need to learn them naturally, out of their context. Even if you learn one new word per day this way, in the long run you'll surely feel the difference.