A straw man is a logical fallacy in which you attack a weaker variation of an opponent's argument. Conversely, a steel man is when you fix up an opponent's argument, and base further argument (pro or con) on the improved version.
It can be hard to construct a proper steel man argument, as changing any factor within the original argument may be seen as either an improvement or a weakness depending on your viewpoint. For example, if I was debating with a young Earth creationist and assumed that his argument was that the Universe was created by God 6000 years ago with fossils and strata intact, this would present me with an argument that I could not disprove, but might also upset the creationist if he believed that this was an argument that God was intending to deceive us.
The best steel men are formed by a consensus between debaters, although in my experience most steel manning occurs when latter debaters restate arguments made elsewhere.
As a general rule, if you do not construct steel men of your opponent's arguments, there is no point in arguing; you have already quit the discussion, and are just preaching in hope that a choir will show up.