How hard Is it to compose a good tune?

Same here. Most of the ideas come when you're nowhere near a musical instrument. Often while I'm having a cig during lunch break at work... all of a sudden there's an idea (which you can't do anything with at that time, bah!). But I'm never too worried about losing/forgetting ideas, coz eventually they'll come back or something else will take it's place.

While you're walking or driving... the movement sort of creates a rhythm inside your head and all of a sudden ideas start to come. When I sit down behind the pc to try some ideas, I usually use either a piano or string sound, depending on the nature of the "song". Often, when you have an idea for a melody, you also have an idea for the sound/orchestration of it, so I tend go straight to that sound. If I have an idea for a string-driven cue, I'm not going to use a piano to try it out.

Oh... and here's a key word in composing : Serendipity. You play/record something, and then "accidentally" change the sounds. Most of the time accidents have to be rectified, but sometimes some really cool ideas come from little accidents you didnt intend to happen. A rhythm that's quantized (synchonised) incorrectly might turn out to be much more interesting that the actual intended idea. Etc.

That's how it is with me. It stinks, because some of my best ideas have been lost that way, as at the time I had nothing to write them down on.