Just realise you'll never get your hair as straight with a blow dryer as you would with a straightener.
First, after washing, towel-dry your hair so it's damp. Then apply a curl relaxant like Phyto Phytodefrisant Botanical Hair Relaxing Balm (see here: http://www.amazon.com/Phyto-Phytodefrisant-Botanical-Hair-Relaxing/dp/B00021DAGI/ref=pd_ys_qtk_rvi?pf_rd_p=186412001&pf_rd_s=center-2&pf_rd_t=1501&pf_rd_i=home&pf_rd_m=ATVPDKIKX0DER& pf_rd_r=1N0EV0ZPN5VRZ03JSAMT ). Put your blow dryer on the hottest setting, bring all your hair to one side and use the hair dryer very close to the hair until it is completely straight, especially at the roots and tips.
Hope this helps

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