Get their attention and make sure where you are standing has space for them to land. Not all dragons will aggro BTW. I've seen one who just like flying around, he looked right at me, and then he flew off towards the horizon and vanished.
I had this experience too, a dragon appeared while I was sneaking near a fort of bandits, the dragon started to attack them, I went in to try to help but the bandits didnt want it of course they just kept attacking me, I could find nowhere to hide and attack so I could see my efforts were fruitless. In the end I said, "Stuff you barbarians, die the firey death you deserve while I watch on the hill side". After a couple of minutes of watching the dragon at work on the fort, he caught sight of me, swooped down and landed right next to me. I had my weapon sheathed and walked around him for about 30 seconds have a nervous squiz at his brilliance, he did nothing at all just kept looking at me. After our cease fire encounter he flew off, sadly he didnt finsh the bandits off he must have gone back to his lair.
Edit: Oh, a dragon fighting tip, if your companion isnt around and you do magika or have a staff of a scroll, summon an atronarch and let the dragon focus his attention on that, keep a distance and fire away with your bow but be on the ready to do a shout or get out your mellee weapon if he spots you. I found in most cases they tend to stick to a cycle of land here, fly there, so the become a little predicable after a couple of minutes making it easier to know when to attack.