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Yes, I agree, the weight of your hair is pulling down on the curls and not allowing them to last too long. You might consider leaving the length on the bottom if you want to retain your long hair, but you might have the top and sides of your hair layered so that setting your hair with curlers will be more effective and help the curls last. Misting your curls with a light hairspray will also help them last longer.
Some well recommended hair products which can add volume and shine are Diamond Shine:http://www.sallybeauty.com/shiny-hair/Diamond-Shine,default,pg.html
Tresemme makes a volumizing mousse which activates with heat, if you blow dry your hair http://www.tresemme.com/Products/Heat/Thermal-Creations-Mousse/
The Herbal Essences line of volumizing hair products is also very good; I've used them and they smell delicious!http://www.herbalessences.com/en-US/hair-products/hair-volume/volume-mousse
If you would like your hair to be curly, you might consider a spiral perm or body wave, and your stylist will know what products to use to protect your hair from any damage during these processes. Also, read this article, below, as it has several recommendations for making straight hair curly. It includes hot rollers, but since you would rather not use those, try some of the other methods.http://www.ehow.com/how_4924437_naturally-curl-straight-hair.html
I hope you get the look you are seeking!
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