I've only worked with fresh material, Marilyn. I think you'll have your greatest success with water color paper from Arches of France. If you don't have a Dick Blick store in your area, you can order it online at
http://www.dickblick.com/produ.... I suggest starting out with item #10011-1008. The paper may seem a bit pricey, but it's a fairly large sheet, so you can cut lots of 5X7 or 8X10 sheets from it.
If you have any daisy-like flowers available, those are almost fail-proof when you hammer them. Of course, the white ones don't work, but colored ones really come out quite well. The same holds true for zinnia petals. Hammer your petals individually, forming a circle, with the base of each petal on one central point. Usually, you don't have to add a center to the resulting flower; the bases of all the petals emanating from the same point will form one for you.
Give it a try and let me know how you do.