This thread is a few weeks old, but I thought I would chip in anyway.
Yes, marian, I share your concern: I've often wished I had a resource that would help me to think in Greek (or Latin) about my everyday life. The Greek Phrase Book that others have suggested is very helpful, but I think you'll find that it doesn't meet all your needs. There are, however, two things that you can do:
1) Keep reading lots of Greek, especially things that deal with everyday life and colloquial language (like Aristophanes, or Lucian). Anytime you come across something that you might conceivably want to say, make a mental note of it. Pretty soon, a situation will arise in which you can think it to yourself!
2) Any time you want to say something to yourself in Greek but don't know the words, don't worry about it: focus on what you can
say. For instance, you don't know the specific verb, but you know what tense and mood it would be in; so imagine what the ending would be. Or, think of something else that you could say about the situation, and say that to yourself. It's kind of like when you're in a foreign country and you don't know the language well, so you can't say everything that you would want to say; but if someone is trying to talk to you, you have to say something
! So just think of any old thing, and say it to yourself in Greek. Then later, when you're reading Aristophanes and you find that someone actually does say that, you'll be really thrilled. Or, you'll learn what you should have said - and you can say it next time