You might be:
Ideas, or imagination
If you want me, you need to be clever, for idiots could never keep me
You need a spark of inspiration to get an idea, if you refuse to think (idiot) you'll never hold on to them
Some people think they can get me with just some water and sunshine/ But don't be silly, I'm not that easy
People think relaxing by the beach is a good way to have ideas come to you, but that does not always work
Please control yourself, if your desire to use me is too strong, I will put your pants on fire!
If you imagine too much, you might end up telling lies (liar liar pants on fire)
Please don't do bad things for me, I might get dirty.
If you have bad inspirations you might have dirty thoughts
But better be quick! For the longer you wait, the smaller I become.
If you don't quickly write down or act on your ideas they fade away.