With all the textbooks / learning materials out there (I've checked out or taught with just about all of them), if I had to pick ONLY ONE, Interchange by Cambridge is definitely it!
It's clear, practical, fun (American) English that people really speak everyday. And it's free (at least the audio / CD part)!
Here's what I recommend:
1. Download them onto your phone or iPod. Listen for 5 minutes everyday. Better yet, listen to them all the time or on the go!
2. Play one dialogue through before stopping or going back. Repeat after (/sound) each line every time you listen to it.
3. If you can, stay with one dialogue; listen and speak over and over until you don't even have to "think about what to say" in the same context.
4. You may find them "easy", with words and expressions that you already know. But the goal is to "master" and "own" them.
5. If you want to go the extra mile, record yourself and play back against the material. Practice some more and repeat the process. The goal: to sound just like the speakers!
"Speaking" is all about "muscle memory" when it comes down to it. Build it up.
So knock yourself out!

*I've used as least 8 idioms in this post. Can you pick them out? Can you use them yourself? Remember: idioms make the natural speech (and informal, everyday writing). Interchange is particularly good in this regard.
Here's the link to the Interchange program - and from the official site too!

http://www.cambridge.org/interchange/audioprogram/
You're all at least at LEVEL 3, so start there with Unit 1!
Kat