I want to preface my reply with a note that I am a new knitter, although every second I am not at work , I am knitting!
I just blocked some sleeves (I knitted them first and wanted to see the post-blocking result) that are 100% acrylic (it is the yarn the pattern called for, ultra soft, and I am still leary of swadling myself in wool that I am still warming up to against my baby soft skin).
First, I wetted a towel and layed it on the ironing board. Then, I layed out the sleeves and pinned them into shape/size. Next, I layed another damp towel over the whole mess and left it over night. The next morning, I had PERFECT sleeves--flat as a pancake.
Maybe my technique will work for you!