Flatter them.
Most dragons love to talk, and even more so to be listened to. Have a very noticeable character engage in parley with a dragon. Only the most bestial would pass up the oppurtunity for some praise. If the character has done his research (as any well-prepared one would), he will know a little about the dragon's exploits. Just drop a few key points into the conversation, and the dragon will probably prattle on for some time about the death of so-and-so knight, or how they sacked that kingdom, or their favorite princess, or something like that. Meanwhile, the other party members, who were never in the lair where the dragon could see them, sneak up under cover of invisibility, or sabotage the cave to induce a cave-in, or something like that, to give them at least a marginal edge.
Think less St. George and more Bilbo Baggins, and you should have a good model for a good low-level dragonslayer. Just be prepared to shell out for a raise dead scroll when the dragon realizes he's been tricked...
Demiurge out.