Lower DR a whole lot, or only give it to a select few encounters at later levels. In a low magic game I ran, the final encounter was against a Marilith. I toned the DR down to DR 5/Good because there was a paladin in the party, but not a chance they would have a cold iron weapon. (The paladin had access to bless weapon and align weapon)
So, if you need to run a monster with DR, there are two things you can do - Remove/massively reduce it's DR to something they could handle. (the magic weapon spell will be the first thing a mage picks up in a setting without magic weapons) OR drop hints to the nature of the creature's weakness long, long before they even encounter it. Example: The old Werewolf-mystery adventure - perhaps at the location of one of the werewolf's attacks the party finds a bloody silver utensil, but the blood isn't from the victim (not that the party can figure that out, but it is a strange clue); it was from the werewolf, who was jabbed by the poor sap who had nothing on hand but a (silver) fork. The party will think the fork to be just a red-herring; a minor random detail. But if they ask what it is made of, then they probably will figure it out.
Random example, I know. In a lower magic campaign, the party has to fight smarter than their monsterous foes, because they can't possibly stand toe-to-toe with them.