My Dear Kholoud,
It�s really tough, sometimes, to keep our students speaking a foreign language in class, but I believe there�s time for everything. Sometimes, it�s necessary, first, to let them share their ideas in their mother tongue, otherwise you may frustrate a moment of theirs to share ideas and your moment to work and teach them something very important.
For example, do you think it would be possible to work/deal with abstract terms or idea, using the adequate amount of time, if you couldn�t count on the students� first language? I don�t think so!
I do believe that we must try to keep our students speaking a foreign language most of the time, but not all the time. On the contrary, you must know how to use the first language as a tool to help them understand a foreign language, not as an enemy.
Suggestions? I have some:
1. Plan carefully your classes and study, also carefully, the material you are going to use or produce. Even doing this, you may, sometimes, find yourself in a difficult situation, imagine if you hadn�t done?
2. Some activities are very useful when you want to set up an introductory session for the topic you are going to teach, preparing them to use the target language as much as they can. For example, word maps, pictures, brainstorming (a very important part of the class, although many teachers disconsider it). Pre-teaching or reviewing vocabulary before starting a topic is the key for success! Word map at the end of the lesson is also very good, because you can see how much your students have improved their vocabulary since the beginning.
3. Teach your students to have a dictionary with them, every class, and, of course, teach them to use it. You, also, may take a dicitionary to your classroom and when they ask for a word in the target language a) ask a student to look up for the word in the dicitionary; b) ask a student if s/he knows how to say the word in English; c) if you, yourself, don�t know the word, say "Let�s look up for it in the dictionary" or, if the word is not important for that moment of the class, go to the student and say " I�ll look it up for you in the dicitionary and I�ll bring the answer next class" and DO IT!
I hope I have been understood by you and I hope I have given some food for thought.
See you!
when it comes to having students remain speaking english in the class. 
