There is a trick to getting there - approach it systematically.
- Research the pose. What should your hands be doing? Your shoulders? Your torso? What is extended? What is bent? To what degree? Study the mechanics of the pose.
- Find poses that work the same muscles and parts of the body. Build a library of poses that do similar things that are not challenging poses for you. Or build a progression - I have to maintain pose A before I can maintain pose B. Get your body to "know" these poses. Regularly practice them with the intention of using the lessons these poses teach you when attempting your challenge pose.
- Practice Satya - truth. Our bodies are very good at "faking" a pose. For example, in Pachimotanasana (seated forward bend), many students roll back on their sitting bones and deeply round their spines to touch their nose to their legs. While you have succeeded in touching your nose to your leg - you have not succeeded in successfully attaining the pose (not very satya). Be truthful with yourself - allow the process to take time. B.K.S. Iyengar speaks of this in his book "Light on Life." He mentions that we should work with baby steps - the world was not built in a day, you cannot save the world in 4 minutes.
- Utilize yoga props. Blocks, bolsters, blankets, straps, slant boards, etc. All of these are utilized in a yoga class as a method of finding the true properly-aligned pose. Some of the best props are props that you already have at home. The wall is the best prop! If you are having problems with triangle pose (or to study your body in the pose -which everyone should do on occasion) - place your hips, shoulders, and arms against the wall. Press each of these parts of the body against the wall. Use it to maintain that compactness that the pose requires (like you are being squeezed between two panes of glass - as most teachers say) - it is also wonderful for getting the chest rotated open. Use the wall for inversions - the wall is an amazing inversion teacher.
- Listen to your body. You will feel intense stretching of muscles in some poses. That is to be expected. Breath into those stretches - and let go. Do, however, note the difference between an intense stretch and pain. If you do feel pain - back off. But try not to shy away from something new.
- The work does not end when you attain the pose. Continue to experiment with it. Perhaps you can set a new goal with the pose - gaining a longer length in the torso, rooting down in the pose, attaining the pose with less effort, or a greater steadiness. Then, research alternate versions of the pose - yoga has a funny way of saying, "have you tried lifting one leg in this pose?" or "now can you add a twist?" Find those alternate versions and play with them - but take it slow. Allow yourself to take those small steps.