Always, always try to use a rest whenever possible. Now I realize there are times when this just isn't possible when a shot presents itself. Sothe only advice I can give you is one that my shooting mentor told me many years ago. First and formost, you will never hold a rifle rock steady shooting offhand. Even the best shooters in the world will tell you this. The trick is to practice, practice, practice. As you continue to practice you will notice that your sight begins to move in a continous circle. The more you practice, the smaller this circle will become. Also, to get a feel for the stance, pick a target, close your eyes and shoulder the rifle. When you open your eyes the sights should be very close to your target. If not, adjust your stance. Do not try to move your rifle. This creates and unnatural stance and, like Lemoyne said, will cause muscle tension - a deterent to accuracy. The next aspect is trigger control. You need a rifle with a good trigger. Only apply pressure when the sights are on the target (moving in that mythical circle). And never, ever jerk the trigger. You will miss every time. The shot should almost be a surprise to you.
Keep practicing and you will become proficient.