Try not to translate what you hear into your native language in your head. It is a very hard habit to get rid of, but it slows down the process of communication and makes concentrating on producing correct English sentences harder, because you are distracted by the rules and grammar of your own language that differ in many aspects from the English ones. It really takes too much time to construe an answer if you have two languages in your head and try to use them simultaneously. Plus, it makes you too hesitant.
Try to relax when you speak. Not only try to get rid of the psychological side of your stress, but also of the physical one. When you are stressed out, all your muscles get tense, including the facial muscles and vocal chords, responsible for the correct pronunciation. That is why it may be hard for you to pronounce English words correctly during face-to-face conversation, although you were able to do it well alone at home.
Do not worry too much if you forget a word; it happens to everyone. In such situation just act as natives do: use so-called “fillers” of the conversation, such as er, um and carry on as if nothing happened. Try also “beating around the bush”: if you can’t remember a given word, try to describe what you have on your mind using other words or definitions.
Remember not to speak too fast. Of course, you need to speak in the natural rhythm of the native language, but if you speak too fast your conversation partners will not be able to understand you properly. Therefore, it is better to speak a little bit slower and be sure that you will be correctly understood.
Keep it short and simple. Do not overuse complicated vocabulary if you can say the same thing in a fewer and simpler words. Also: do not build too long sentences. Firstly, they might be too difficult for your partners and secondly, you yourself can get lost in them and forget what you wanted to say. This would be quite embarrassing, so try to avoid such situations at all costs.
Do not use too much slang language or idioms. If you do not talk with a native, there is a high chance that you will not be understood. And even if you do talk with an Englishman, there is a possibility that you will misuse an idiom and will not be understood properly. The safest option is to use only the most common idioms, which meaning is clear to us.
Remember about the intonation. Every language has a different intonation and using the Indian one in the English sentences may cause many misunderstandings. The intonation and accent are one of the most difficult features to learn in every language. You need to train your ear to be able to hear the nuances. You can find English speaking tips and information on intonation on many websites or use lessons placed on YouTube:
Train all your English skills: listening, speaking, grammar and vocabulary. Do not forget about any of them, do not train only one separate. Any single one skill cannot be developed in isolation, because every language is a complex structure build from many little elements. Every unit has its place and is connected with others. Therefore training only one skill and forgetting about others does not have much sense.
If you feel such need, you may use books that are written to help foreigners to improve their spoken English and contain many English speaking tips. Below follows a list of books worth recommending:
Essential English Grammar by Murphy (Cambridge)
Spoken English by R K Bansal and J B Harrison
Pronounce It Perfectly In English (book and three audio cassettes) by Jean Yates, Barrons Educational Series
English Pronunciation For International Students by Paulette Wainless Dale, Lillian Poms.