You just need to learn how the syntax works for reference-style links. Something like:

[Hacker News][hn] is a website.

[hn]: http://news.ycombinator.com/

You should generally use these all the time when writing documents you intend to maintain. For quick one-offs like blog comments, you can instead use:

[Hacker News](http://news.ycombinator.com/) is a website.

Once you actually use markdown for non-trivial documents, it is very easy to remember its syntax.

That's interesting, because I've heard that link syntax is one main reason people prefer Markdown to Textile. One issue with Textile-style links is that something like

  "Foo Bar":http://foo.com/bar.

is ambiguous, and the result depends on the parser. It could end up as

  <a href="http://foo.com/bar">Foo Bar</a>.

or it could end up as

  <a href="http://foo.com/bar.">Foo Bar</a>

The former is probably what you mean most of the time, but then there's no way to link to the (valid!) URL "http://foo.com/bar.".

This is my biggest problem with Markdown - I still use it because I'm too lazy to switch, but every single time I have to insert an image I have to check the syntax page to see if the title goes in parens or if the link does (while neither makes sense because regular links use brackets for both).

So while I use Markdown, I recommend Textile just so you can avoid this issue.