Rabih, Algeria
The "schwa" as you say is a weak vowel, and it is the most frequent sound in English. I tell my students where they can normally find it, for instance in many nouns ending with the letter "a". They are pronounced with the schwa , for example America, Canada, sauna, sofa ...etc

Aboumeriam, France
Hello Seyda
I teach my students that syllables are like light bulbs. The word "whisper", for example, has two syllables, so two light bulbs. But I have just enough power to light one bulb. The other is left "powerless" or "reduced" because of the lack of energy. In the case of the verb "whisper", I light the first syllable and neutralize or reduce the second.

Nicole Anderson, Germany
In Turkish you have a letter which corresponds perfectly (in my opinion) to "schwa": I as in the sound at the end of the name ASLI.

Michele Hunter, Germany
I have a clear image when talking about "schwa" burned into my memory from my CertTESOL course. Our pronunciation teacher physically demonstrated the weak, collapsing schwa sound by slumping her shoulders and pulling an exhausted-looking face whenever a word featured the sound. It not only makes the class smile, it reinforces the point of weakness.

Lia Cross, Australia (teaching in China)
Seyda, try the good old fashioned method of getting the students to place a hand in front of their mouth when pronouncing a word with the weak syllable in it.
Stressed syllables should produce a soft puff on your hand and unstressed should not. The verb 'whisper', suggested by Aboumeriam from France above, is an excellent word for demonstrating this.
Good luck,
Lia

Mal James, England
Hi Seyda
If you want a really good explanation and some exercises to Illustrate go to the BBC Learning English website for a radio programme and downloads including a good pdf file and audio file.

http://www.bbc.co.uk/worldservice/learningenglish/multimedia/pron/schwa/...

I have used this file while teaching it to students who had learned English in other countries and thought I as a native speaker was using slang until I played the sound file. You will also find programs on the connecting 'r' and the extra sounds 'weggs' and 'yeggs.'
Mal

Carlos, Colombia
In my case, I have always made my students compare the existing difference between "away" and "way". That extra soft sound in the first sample gives them a good idea of the sound. Then, I use words like "cinema" to show the sound in an intermediate position or a simple sentence with "to" to show how it changes into a schwa sound.