What is the most successful way of teaching English language as a foreign language? Is it by just using English language all the time?

The most successful way is to vary methods and techniques as much as you can.
Just make sure it is the right decision by considering the learners, their level of English and their interests.

It is not just the teachers job to speak English all the time, even if he does, it is not in the goal of making students hear him use this language all the time.
What a teacher must do, is try to influence the students in a way to make them use English too, it is his job of letting students not only communicate in English but also use it to start thinking and coming up with ideas, this will help students to use the foreign language they are learning

Depends upon the purpose of learning English. If it is only spolen then only speaking practice can suffice. If it is for an Academic purpose then just speaking practice is not sufficient. What I do is to prepare a work sheet with few sections on daily basis. It contains very interesting tasks like idiomatic phrases and grammar parts etc. But all the tasks need conversation within the class. In English language class I never allow the use of any other language. So it creates an environment favorable for the students to converse. They are encouraged to speak in English even if it is incorrect. All the students in the group get opportunity to discuss and converse and ask questions and answer questions and write small assignments. They really enjoy my class and remain punctual.

In teaching of English language as a Foreign language, one size does not fits all! There is no one specific way of teaching English language successfully. The first thing one has to do is to consider the learners' needs of the language. Then, comes the appropriate syllabus designed based on their specific needs. This syllabus should vary in terms of successful knowledge transmission and language engagement. 

The best way to learn a language is to speak it.. Teachers should not only speak English in the class they should also discourage the students who speak their mother tongue. This of course requires contstant efforts, encouragement , patience and motivation . Some students feel that English is the language of their teachers. Teachers should create an environment in the class wherein the students don,t feel hesitation. 

In my point of view the best answer is to use the communicative approach in which the target language is very important for class communication not only for students , also authentic language is very useful i.e using language in areal social context

From experience the best way to teach a second language is to make it fun, which means involving the students in group activities , team work can make things easier because some students might find it easier for them to ask another student for help rather than asking the teacher.

You forgot to mention the age group which perhaps is the key factor. For instance if students are young learners, you will need to have a 'bag' of activities at hand so you can keep them motivated by engaging them in different activities such as songs, role plays and academic games which perhaps is Total Physical Response technique. Same goes for adults. A good teacher always keep vareity of plans stored in his mind to keep the classroom environment alive. 

By motivating the students through different techniques to speak English rather than mother tongue.

I think to teach English as a foreign language u shouldn't only speak English all the time but u should also motivate your class to participate and try to speak too...and that will come by making the class more interesting...u can do that by using pictures as the visual aid is important to adults and children in learning....also u have to u gestures to help make them understand....some students maybe too scared of talking especially the true beginners and so to motivate them and help improve their language u have to gain their trust and that could be done by always smiling, using motivating words, having patience and correcting mistakes nicely....you could play games to help engage all students and make them interested.....games vary according to age and level.... I always get them chocolate when they do something good or when i feel improvement and it makes them happy whether they r adults or kids....there are many ways u just have to understand your class and know their interests...

Student centered teaching is most appropriate approach to teach English as a foreign language.
Teachers should speak English all the time and the should encourage their students to do so.
But the most important technique is to let the students lead the class which is the best way for them to practice communication.