Because most of my teachers are not on italki, I want to address what I have found as the advantages of having tried many teachers instead of just picking one and sticking with them.  I do have a few teachers that I have had more than 50 lessons with, but many others that I have had only a few lessons with.  

With each new teacher, there is a getting acquainted process.  Instead of seeing that as a waste of time, I see that as an advantage, because the early conversations can be similar to what I would most likely need to be able to talk about with strangers or new acquaintances who speak my target language.  It has been an advantage for me to have had probably at least 30 different people that I have repeatedly had conversations repeating isuch basics as what I do for a living, where I live, about my family, about the weather, etc.  Personally I have needed this repetition to become fluent and comfortable in talking about these common subjects and be prepared to answer the type of questions that new people are likely to ask me. Repeatedly going over these things with one teacher would get boring, but with new teachers it is like ¨real life conversations.¨ 

I have found that different teachers have different strengths.  Some who are great conversationalists are not so great at teaching grammar.  Some are more willing and able to help with pronunciation than others.  Each teacher has different interests.  I like to have teachers teach me about things they are passionate about but that I know little about,  while using the target language. I prefer to use a team of teachers, with me deciding who is my expert in which activity. I have also found when something confuses me it is helpful to have it explained by more than one person. Each teacher explains things differently and having two different explanations usually clarifies things for me. 

Get native speaker teachers of the language you want to learn. There are tons of people asking to improve their English for their language in exchange.

What you basically look for is to speak, right? Get as much native friends of the language you seek to improve and be responsible when it comes to teaching back ;)

An advantage of having a teacher in italki is that there are excellent ones who can really offer you the time and right paths to get you committed. There are also people who are not teachers but also have the time and friendship.

I say get people who can help you speak, teacher or not. Use the internet, translators, texts, books, movies, foreign friends, all this you can do at your own pace!!

Peace ;)

Allow me to make a couple of suggestions.

1  Read teachers' profiles (make notes if you have any questions to ask the teacher about his/her qualifications)

2  Set up inexpensive 30 minute Trial Lessons with 2 or 3 teachers. Tell the teacher what you need and what your goal is. Then listen to see if the teacher can tell you how he/she can help you meet your needs and goals.

3  Choose the teacher that you think will help you the most.

The 3 Trail session approach may help you save money and also help you to find the best teacher for you.

Good topic Sepideh...

I usually switch teachers if I feel the person is not making as much effort as before or if I don't like their teaching style/personality after an initial meeting. 

The advantages are that you are more likely to find the teacher and style that bring out the best in you, you hear a variety of voices and accents, and learn more about how you learn best.

The disadvantages? It can be nerve-wracking to start over with a new teacher, you can waste time repeating myself introductory information, you might cover topics you've already learned again if you don't clearly state what you have studied previously, and you will deal with disappointment. Some teachers are nothing like their profile persona, and this can be a let-down, because at least with a regular teacher you know what to expect. 

If I'm happy with a teacher, I'll take many lessons and continue until I need a different approach to take me to the next stage. If the teacher is good but not great, I will take lessons but continue trying out others. In some languages, I seem to have a very easy time connecting well with tutors, and in others, the personality and method are consistently not what I'm looking for. I don't know whether it's a question of language mastery or culture.