If you want to incorporate more advanced asanas, ask your teacher if he or she thinks you're ready. If you're practicing on your own, assess your progress and limitations as objectively as possible before advancing. It's more important to master the foundations of yoga that apply to all hatha yoga asanas than it is to "master" the postures. If you've been practicing regularly for a year with focus, proper alignment, weight distribution, and breath control and can stretch to your limit in a posture on either leg without losing your balance, that means your muscles are evenly developed around a skeleton in alignment and balance with the earth. That's a good sign that your body is physically and mentally ready to take on more advanced asanas.
When you add asanas to your practice, you should have a clear understanding of each step. If you have any physical limitations or injuries, learn the modifications before you begin. Remember to maintain alignment, proper weight distribution, and breathing as you approach the posture slowly but firmly. If you respect your limits, you're less likely to be injured. Stop and hold the posture when you can't move further into it without sacrificing alignment or balance. Even if you don't feel like you're reaching your potential, remember that proper alignment and focus improve flexibility, balance, concentration, and will power. Your body will develop strength and balance evenly, preparing for more advanced postures.
Whether you master the asanas or not, there are countless benefits to attempting and working on more difficult postures as you continue to practice. If you are practicing on your own, you may find it helpful to work periodically with an experienced, knowledgeable instructor, either in a class or privately, who can help take you to higher levels.