Listening to music doesn't ruin the fun of making a level. Sometimes I am really in the mood to make one, but sometimes I have no inspiration so I don't make one.

Here are a few tips that might help.

1. If you don't have inspiration to make a level, look at some famous levels. It helps.
2. Gameplay is the most important, although I am a sort of eye candy freak who loves great eye candy. Of course the other things are important too, like the ammo placement. It all depends on the level type and if you make a SP level, the different tilesets have effect on the enemies you will place in the level.
3. Search a music that fits your level well.
4. Don't upload a level to J2O when you just finished it. You need to find beta testers who will try to find bugs or they tell you what can be improved.
5. I usually start with the layout, than I'll do the placement and I'll do the eye candy at last. But of course, you can do it in another order. It's just your choice.