You can get flat prints if you build a print dryer. You need corrugated board and Pellon. You could also use blotter paper. Here is a description-
One thing is to make sure you have squeegeed the print so no water is left on the surface. Also keep the wet time down to as little as possible.
If you have a clamshell dryer, putting a dry print face up on it will take most of the curl out. I usually take a stack of prints and put them in a clamshell dryer, then turn the heat all the way up. When it gets hot, I turn it off and let the prints cool down. When done the prints will be fairly flat.
You can also pull the print over the edge of a table. That works if you are desperate....
Hope that helps you.