There is a chance that someone else will trade for your guy, which is why it is preferred to trade with a friend, but it can work.
You could just offer it for a Lv. 9 or under legendary if you've seen one. (Note: Don't do it with Mew, Jirachi, Dialga, Palkia, Giratina, Manaphy, Phione, or Regigigas.) Either that or an evolved Pokemon that can't be obtained at those levels. IIRC, even if you don't make a trade, after 24 hours, the Pokemon will evolve anyway.
Edit:
Also I have a question about the HG/SS breeding mechanics. If I wanted a Milotic with 31 IVs in HP, Def and SpDef with a bold nature and I have a Ditto with 31 IVs in HP and a Milotic with a bold nature and 31 IVs in SpDef and Def, would I have a better chance of getting the Milotic I wanted by giving my current Milotic either the Def power item, the SpDef power item or the everstone?
I'm fairly certain you would want an Everstone since there is a 1:24 chance to have Milotic pass the Def and SpDef which means there is a 1:48 chance to get the correct IVs and Nature. There is a 1:25 chance to have the nature you want to appear and a 1:6 and 1:4 chance for Def and SpDef to be passed down respectively so you would have a 1:100 and 1:150 chance to get the Milotic you wanted if you used Power Belt or Band respectively. I'm fairly certain this math and the probabilities are right.