Dick,
There are many ways of flattening fiber based paper. For me, an old fashioned print dryer (glazer) is the best way to go. Be sure to put the paper on the steel plate emulsion side facing down for glossy and emulsion side facing canvas for mate or semi-mate.
Glazers cannot be used with RC paper, so they have kinda shifted out of fashion. It's quite difficult to find a new one nowadays, but you should be able to get a second hand one for a few bucks. Here is one of the last few models for sale on Freestyle Photographic supplies:
For alternative methods, check these threads on Photo.net:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=008rRb
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=000PPh
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=002Vpm
Cheers,
Sebastien