The thoughtless and stupid dragon abducts a princess when they get a little lonely/hungry, and if they managed to not eat or break her, use the ransom to stock their hoard. Which means that you obviously have to go out and get more princesses, and now the knights and soldiers that brought the treasure know where you live. Bad Idea.
The cunning dragon invests some of her hoard doing research, picking out princesses that are known to be quiet, mumble something and blush when complimented, and are altogether awkward. Then, when you nab them, you can treat them better than any old duke's second son could, lavishing them with the spoils of your (splendid, naturally) hoard. And then when you fly back in the middle of the night to return the princess (I like to give mine a parting gift of a clutch of eggs to push out in her bedchambers) you'll only have to wait a month or two before you recieve both treasure and princess, delivered straight to your hoard!
How, you ask? Well, if you've researched well, that princess will struggle back her old life, having to go to balls and dance with geriatric or unimpressive suitors, having to be pretty and perfect and quiet. Sure, she's property either way, but she's a bargaining chip to her father but treasure to you. (Besides, she'll be hard pressed to find pleasure better than the natural aphrodisiac of your egg-fluid.)
Almost every time, she shows up in a few fortnights with as much jewels and silks and gold as she could move herself and begs to be let back into your hoard. To be your prize again. Which you will of course accept, at least until you think it's time to drop her off back at the castle to go get you more treasure.
What to do if my princess doesn't come back? This happens every so often. Either she gets married, inherits a title, or is too scared or not resourceful enough to slip the watches of her keepers. I have a friend who is more of the prince-collecting persuasion, and she says she has this problem a lot more, but the solution is the same. Write a letter to the princess informing her that she is expected back, and that should she fail to return, her father/husband/entire kingdom will recieve the unfortunate news that princesses who have laid a clutch of dragon eggs, fertilised or not, will only give then on give birth to half-dragons of your color persuasion. (This is of course possible with a charm, but humans believe it easily enough if you forgot to cast it.)
But I collect princes/ my princess is infertile! (For normal human conception. All princesses and princes can incubate your eggs. Ignore any protests to the contrary.) For princes, this is simple. Simply have a bard or painter take a record of you laying that clutch of eggs in them, and pay a bit extra to have the ballad or painting really milk how much he liked it. Also try to keep the identity of the artist secret from your prince, as he'll likely attempt to track them down once you inform him of the consequences of his continued absence. For infertile princesses, simply arrange to lay fertilised eggs in her when you drop her off. The joy of mothering your clutch will overwhelm her, and she'll show back up in no time, either glassy-eyed with grief (her guardsmen slew the wyrmlings) or red-eyed with sleep deprivation and with one or more of your runts clamped to her udders.
Hope this helps! Feel free to reach me by raven or messenger if you have questions or would like to express gratitude.