Well, if you want to increase your store of anything, you've got to start collecting. Maybe you should start thinking of your vocab' as a collection of useful and or beautiful words. If I was very rich I might start a car collection - I'd have a few beautiful sports cars, a couple of limo's and a few more practical cars - for off-road driving or for transporting stuff. With words it's not dissimilar - maybe the proportions are different as we need loads of practical words, but if it's a hobby we should have at least a few 'beautiful' words. By beautiful I mean words which maybe sound nice, have weird meanings, are translations of Polish slang - whatever appeals to you. I studied a bit of French - I can say 'My name is Andy' and say 'how are you' but I also remembered the French for potato - 'pomme de terre' as it seemed really cool to me - the direct translation means 'apple of the earth'. If you've got some beautiful words to show off with you also feel more confident in your speech.
So we need to start collecting our words - when, how and where? I won't ask the questions 'what' or 'which' because every word is useful if we meet it in a natural way (an unnatural way would be reading a dictionary). If you have to ask the question 'why' then you should give up now(.
Firstly 'When' and 'Where'?
Easy answer - whenever and wherever you can! Be proactive. When you meet a new word at work, in your English lesson, on TV, in a song, in a book etc you should underline it, maybe note it down in a notebook or even in your mobile phone. When you get home transfer it to your 'Excel' table of word families. See another of my articles -
FCE - The best way to revise for the Reading paper
Write it down - use the computer to make word family tables - print them out when you've done one and keep it with you so you can read through it in a spare moment. I used to keep a very small notebook in my pocket of new Polish words and phrases and look at it when I was in the bus.
Some mobile phones can read such documents from the computer - use one if you have one! Why not get your mobile phone to give you a reminder every day of the new words you have - just set it up like an appointment or task in your phone. You could also do this with your Outlook or other email program. There are websites you can sign up to for a word of the day email - just google 'word a day' - you'll find hundreds.
Why not write example sentences and get them checked by a teacher if possible - just to make sure you are using the word correctly. Or just copy them from a dictionary - you should have decent English - English dictionary to help you in your studies (Ideally 70 000 words plus, but 40 000 isn't a bad start). This is helpful because you can see what other words are used together with your new word - so-called collocations.
Most MP3 players have Dictaphones - why not record the words and listen to them - it's good to hear yourself talk - even if you're just reciting a list of vocabulary - this also gives you confidence.
I'm sure that if you are proactive and imaginative in your collecting you'll enlarge your collection in no time at all.