If you raise your shoulders and lock your arms your whole body is controllable through your wrists - so you don't want that.
Your arms should be very slightly bent. When you are trying to extend, imagine you are reaching out into the distance e.g. trying to grab the opposite wall; it should feel a bit like the stretch you do when you are yawning. What is happening is that your triceps are keeping your arms in this position (so you do have to use muscles), but the atagonistic muscles - the bisceps are not coming in to play. It is then your hips and centre that are pushing forward, and your arms are just out in front. If you find you arms collapsing it is because you are pushing in with your centre, but the arms aren't moving with you - it is far better to just stay where you are and feel the pressure, rather than collapse.
Like everything in aikido, you just have to keep training and eventually you'll understand!
There are excerices you can do to help develop relaxed shoulders while extending. Plenty (about 2-300) of bokken cuts on a regular basis is useful. Not only does it build up your back muscles, but your shoulders generally aren't strong enough to do this many cuts whilst tense, so you start to use your body properly.