Alpha has been my personal key in using the LOA.
It need not be your personal key. I think that different people will be inclined towards different methods. Eg some people will do very well just by managing their emotions effectively. Others will do very well just using positive affirmations or writing out their goals regularly.
But for me personally, alpha has been the key.
Ideally you want to get into a state of deep relaxation, yet retain mental alertness. It is a state that can arise from hypnosis, or prayer, or meditation, or through methods such as progressive muscular relaxation.
Just getting to alpha and staying there for some time, eg 15 minutes, is in itself a beneficial exercise, even if you are not manifesting anything. Basically it feels very good.
If you do a bit of googling about stress management workshops, you'll see that the exercises they teach will actually lead the person into alpha. They may use methods such as progressive muscular relaxation, or deep breathing exercises, but the effect is the same.
As far as the LOA is concerned, the whole idea is that (1) you go to alpha, and (2) only then do you start thinking about your goals and intentions.
Unfortunately it isn't always easy to get to alpha. It takes some practice and sometimes the environment poses challenges. Eg because I have two young kids and they can be quite boisterous and noisy, my home isn't always conducive for getting to alpha.