I really enjoy cooking eggs for breakfast, and sometimes I'm feeling fancy enough to add veggies into my scramble. However, I'm never sure how to prepare the veggies to put into the scramble. If I don't cook them, usually they are too crunchy or tough (and sometimes make the scramble watery), and don't taste good with the eggs. If I try to cook them before adding them, they usually come out too dry/overdone. I figure this might depend on which veggies I'm cooking so here are my common favorites: tomatoes, bell peppers, mushrooms, onions (suggestions welcome!)
To make my question more specific (and hopefully more answerable): how can I prepare the veggies so that they are not crunchy, but not over-cooked?