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Lots, and lots of water. Staying hydrated is key to staying vocally healthy.  Stay away from cold water. It tenses your vocal folds up when they need to be relaxed to hit those notes. Tepid water is best.  Milk coats your vocal folds. It may feel good to drink, but its not good for your voice. If you are feeling a strain, don't drink liquids that are too hot. Warm tea (with a bit of honey is fine); thin, room temperature liquids are best. You know those girls you see in Victorian pieces in the movies and on television? That's not a bad place to start.  If there's a back to your chair, don't use it. Keep your back straight and your arms loose. Don't suck in your stomach. You're breathing from it, remember?! Keep your body as relaxed as possible. Relaxing your voluntary muscles makes it easier to relax your less-than-voluntary ones. When you start feeling yourself reach vocally, do it physically. You'd be surprised how the physicality helps.  Start with your arm by your side at the beginning of your siren and make a circle as you go, reaching as high as you can vocally and physically simultaneously. Think of throwing a frisbee when you do trills and high warm ups. Some coaches advocate literally pressing down when you're warming up with arpeggios and hitting your vocal breaks. The idea is that pressing down with your hands reminds you to keep your larynx low. Simply enough, a professional's guidance will be the quickest way to seeing the results you want. However, keep in mind that every vocal coach is different and you'll obtain different results with each one. Ask your potential coach questions about their own training, what techniques they employ, and what genres of music they'll be teaching you to start. Some coaches might give you a very poppy sound and others very classical; yet others a happy medium.

Summary:
Drink water. Practice good posture. Use your arms. Get a vocal coach.