Someone appears after you mention them. "Speak of the Devil" is short for "Speak of the Devil and he shall come". It was believed that if you spoke about the Devil it would attract his attention. That's why when you're talking about someone and they show up people say "Speak of the Devil".
Act quickly while the opportunity is still available. Blacksmiths working iron by hand heat the iron in a fire to red-hot making it malleable. The Smith removes the iron from the fire and shapes it with blows from a hammer. They need to work quickly before the iron cools. Once the iron is cool, it becomes brittle and the opportunity to hammer it into shape has passed.
If you want the job, you need to strike while the iron is hot.
Not getting a fair share. Many female mammals have multiple rows of breasts, for example dogs. Typically the rear most pair of breasts is smaller and less developed than the rest. Hence a pup nursing from the rear most breast is likely to receive less milk than other nursing pups. Hind means rear most. Tit is slang for breast.
Since the introduction of Windows '95, Apple Computer has been sucking hind tit.