Designing experiments it a very cool, if sometimes a little daunting challenge. More often that not I might design an experiment and not get the result I might have expected, this is why it is important to always go in with an open mind and never try and make the data give you a result you want. I might not get results sometimes that are meaningful so I have to rework the method to include controls to make sure all the parts of the experiment are working properly so that I can be sure that any data I get is real.
A lot of the time I will read other peoples work and see how I can alter their methods for the experiment I am trying to do. This can save a lot of time, providing the other person has written down all their steps correctly, other wise you have to start from scratch yourself.
There is no better feeling to me then when I design an experiment and I am confident that my data gives a meaningful result, especially if I have been working on it for a long time.