To excel in speaking section, I have only one suggestion “speak your heart out”, speak as much as you can. Try to elaborate as much as possible. Focus on phrasing grammatically correct and coherent sentences. You can try practicing in front of a mirror if you are unsure about your posture. Don’t be much worried about your accent,also don’t try to copy a particular accent. Be calm, confident and maintain proper eye contact. You will get one minute to think about the subject they will give you, utilize the time wisely. Write down the keywords what you want to convey. Sometimes it does happen that we think lots of stuffs to speak about back in our mind, but when it comes to execution we run short of words. Please make sure it doesn’t happen. Once again be relaxed, confident while you speak. I am sure you can nail those 15 minutes.
One small tip: If you feel skeptical about your voice, accent and want to rectify your mistakes, please record each of your practice session and analyze them.
Hope it helps. Cheers!!