I would not advise practicing lucid dreaming in the shower. 
Anyway, There are 3 things you want to learn about to succeed with WILD:
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Deep relaxation. You can find my beginner progressive relaxation technique on pg 2 of this thread.
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Enter a trance state. Now you might already be in a trance after you complete the above exercise. If not then return to concentrating on your breathing. As you inhale imagine you are breathing in a powerful anaesthetic that relaxes all the muscles and nerves in your body. As you exhale you are breathing out all your worries and tensions. Continue until you enter a trance state.
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Now you should be in a trance. From here you just have to let go a little bit more. Continue letting go of all your thoughts you need only hold on to a small fragment of you consciousness to crossover. You may well even experience a brief blackout but that is ok. Now pick something to concentrate on: you can try to visualize and object and just be aware of it, counting, focusing on a point on your body etc. It really does not mater what you focus on so long as it does not require you to think about what you are doing. The important thing you need to under stand is that any thoughts or feelings will bring you back to your waking state. With practice you will find that either you get pulled into the dream or the dream will come to you.
This all sounds hard but, it really is not. It is just something you have to practice. I can enter a trance very quickly and you can learn to as well.
Well I hope that makes sense to you and helps. 