1- Look for good iron gatherers and train them in priority.
2- Make full use of the half price training events which come around every month or so, you can save a ton of iron this way.
3- It gets easier the higher level you are.
Here are the best dragons for iron gathering (stats in parenthesis at max level):
Wood for iron: Stonewall (450M for 1050 in 2h37); Dustbrawler (499M for 1051 in 1h03); Trailtwister (400M for 840 in 13h07); Aurvandil (250M for 315 in 1h12); champion Windgnasher (250M for 262 in 1h18).
Fish for iron: Schmoozer (425M for 1050 in 1h45); Sweet Wraith (400M for 840 in 13h07); Rattling Smokebreath (100M for 210 in 2h06); Dramillion (450M for 672 in 1h03).
A few premium dragons are pretty good too. And last but not least, the little chickens have the absolute best conversion rate - 1 fish for 1 iron - but their gathering speed is slow.
Good luck and have fun!