I've found that having ideas isn't the problem for me. Writing them down and remembering them is. Ideas often come to me while driving or just before falling asleep and I've lost count of how often that killer concept has eluded me in the morning. I've (just about) got in the habit now of dropping everything when an idea comes to me, even if it's just to scribble one key word or a single sentence.
I never
try to write (although perhaps I should). I seem to get very powerful 'writing moods' where I can barely type fast enough to keep up with the stream of words coming out of... somewhere. My record is something like 7500 words in a single session lasting until 4am but I guess that's probably nothing to the pros.
I've also been advised on many occasions that I agonise too much and work too hard on finding something brilliant and original (and inevitably failing). Some of the best things I've ever read have actually had extraordinarily simple concpets, it's what happens to bring that concept to life that makes them popular. Let's face it, JK Rowling is a multi-millionaire on the basis of 'A young boy goes to wizard school.' How much more simple can it get?
Jon