I have found the size of the exacto knife blade makes a difference. The crown for example worked well with the 1" wide blade and the small 3/8" blade worked well to smooth the edges. The large blade also worked better for the bullet helm, weapons and smaller details work better with a smaller blade. The results of my cutting can be seen here:
Just showing a little of what can be done without a dremel, not everyone has one of those. A steady hand and slow steady pressure works best when cutting thicker elements like the crown. I have more stuff I have done, but no pictures yet.
Edited for typos... sooo many typos...