I keep back the last 5-6m of a roll of heavyweight matte (not gloss or satin!) paper as a 'de-curling' device. Unroll a good length, then place the offending curled print face up, then roll it inside so it curls the other way. Leave for a while and the print will come out curled slightly the other way. It then flattens out as the print tries to re-curl itself. A few trials and now I get a flat print every time. Don't use gloss or satin as the 'de-curler' because the print image will stick to it. Occasionally I buy a roll of cheap heavyweight matte paper especially for this purpose. I find that matte prints de-curl very quickly, while gloss/satin can take as long as overnight.