Sharing is a very hard concept & totally agree with the PP, adults are actually far worse at it than children when it comes to things we "value", yet we expct children to share everything they own.
Practicing taking turns is great, also if you can picking things that are easier to share, such as blocks & plastic containers from the kitchen rather than toys that there is only one of.
When do you think it's time to bring in the time out/naughty corner?
My answer is if you think you as an adult would change your behaviour (or your spouse would change their behaviour) by being put in the naughty corner, then try it with your child, but,at the end of the day the naughty corner only makes a parent "feel they do something" & while it *can* stop a behaviour, it doesn't help the child learn why what they did was wrong & what to do better instead if they aren't given opportunities to learn. If the children around me whose parents "swore by naughty corners" when they were preschoolers are anything to go by, by the time they get to an older child (say 6yo) they become really good at avoiding getting caught, rather than knowing how to behave.
IMHO the best way a child learns is two ways 1) what is modelled to them by parents... the more "stubborn" a parent is, the more "stubborn" a child learns to be & 2) practice, practice, practice... it takes on average, around 12-14months for a child to learn to walk & around 2+ years for them to put 2-3 words together, it takes a life time to learn life skills. The best way to learn sharing is to practice, to allow him to be upset about it, show empathy when he is upset (not chastise) & try again. The more you chastise the less likely he is to learn anything from the moment (no one learns when angry) & is more likely to feel angry that you get cross when he is confused as to why things are happening different to how he wants them to happen in his mind... the tears are generally from not knowing why what you want is different to what he wants, rather than him being stubborn...at 18mo a child thinks that everyone is thinking the same as they are & they don't get why it doesn't happen their way, the brain wiring for empathy isn't their yet, which is why they need the parent to support their upset not chastise it.