How do I center a Bootstrap div with a 'spanX' class?

I use bootstrap and am trying to center a div(span7) like that:

<div class="row-fluid">
    <div id="main" class="span7">
        Lorem ipsum dolor sit amet
    </div>
</div>

my css code is:

#main{
    margin:0px auto;
}

But it isn't set at the center. How can i set center?

EDIT

span7 is now col-7 in Bootstrap 4 and row-fluid is deprecated(Instead of this, use container-fluid and row).

Twitter's bootstrap .span classes are floated to the left so they won't center by usual means. So, if you want it to center your span simply add float:none to your #main rule.

CSS

#main {
 margin:0 auto;
 float:none;
}

Incidentally, if your span class is even-numbered (e.g. span8) you can add an offset class to center it – for span8 that would be offset2 (assuming the default 12-column grid), for span6 it would be offset3 and so on (basically, half the number of remaining columns if you subtract the span-number from the total number of columns in the grid).

UPDATE Bootstrap 3 renamed a lot of classes so all the span*classes should be col-md-* and the offset classes should be col-md-offset-*, assuming you're using the medium-sized responsive grid.

I created a quick demo here, hope it helps: http://codepen.io/anon/pen/BEyHd.

If anyone wants the true solution for centering BOTH images and text within a span using bootstrap row-fluid, here it is (how to implement this and explanation follows my example):

css

div.row-fluid [class*="span"] .center-in-span { 
   float: none; 
   margin: 0 auto; 
   text-align: center; 
   display: block; 
   width: auto; 
   height: auto;
}

html

<div class="row-fluid">
   <div class="span12">
      <img class="center-in-span" alt="MyExample" src="/path/to/example.jpg"/>
   </div>
</div>

<div class="row-fluid">
   <div class="span12">
      <p class="center-in-span">this is text</p>
   </div>
</div>

USAGE: To use this css to center an image within a span, simply apply the .center-in-span class to the img element, as shown above.

To use this css to center text within a span, simply apply the .center-in-span class to the p element, as shown above.

EXPLANATION: This css works because we are overriding specific bootstrap styling. The notable differences from the other answers that were posted are that the width and height are set to auto, so you don't have to used a fixed with (good for a dynamic webpage). also, the combination of setting the margin to auto, text-align:center and display:block, takes care of both images and paragraphs.

Let me know if this is thorough enough for easy implementation.

Update

As of BS3 there's a .center-block helper class. From the docs:

// Classes
.center-block {
  display: block;
  margin-left: auto;
  margin-right: auto;
}

// Usage as mixins
.element {
  .center-block();
}

There is hidden complexity in this seemingly simple problem. All the answers given have some issues.

1. Create a Custom Class (Major Gotcha)

Create .col-centred class, but there is a major gotcha.

.col-centred {
   float: none !important;
   margin: 0 auto;
}

<!-- Bootstrap 3 -->
<div class="col-lg-6 col-centred">
  Centred content.
</div>
<!-- Bootstrap 2 -->
<div class="span-6 col-centred">
  Centred content.
</div>

The Gotcha

Bootstrap requires columns add up to 12. If they do not they will overlap, which is a problem. In this case the centred column will overlap the column above it. Visually the page may look the same, but mouse events will not work on the column being overlapped (you can't hover or click links, for example). This is because mouse events are registering on the centred column that's overlapping the elements you try to click.

The Fixes

You can resolve this issue by using a clearfix element. Using z-index to bring the centred column to the bottom will not work because it will be overlapped itself, and consequently mouse events will work on it.

<div class="row">
  <div class="col-lg-12">
    I get overlapped by `col-lg-7 centered` unless there's a clearfix.
  </div>
  <div class="clearfix"></div>
  <div class="col-lg-7 centred">
  </div>
</div>

Or you can isolate the centred column in its own row.

<div class="row">
  <div class="col-lg-12">
  </div>
</div>
<div class="row">
  <div class="col-lg-7 centred">
    Look I am in my own row.
  </div>
</div>

2. Use col-lg-offset-x or spanx-offset (Major Gotcha)

<!-- Bootstrap 3 -->
<div class="col-lg-6 col-lg-offset-3">
  Centred content.
</div>
<!-- Bootstrap 2 -->
<div class="span-6 span-offset-3">
  Centred content.
</div>

The first problem is that your centred column must be an even number because the offset value must divide evenly by 2 for the layout to be centered (left/right).

Secondly, as some have commented, using offsets is a bad idea. This is because when the browser resizes the offset will turn into blank space, pushing the actual content down the page.

3. Create an Inner Centred Column

This is the best solution in my opinion. No hacking required and you don't mess around with the grid, which could cause unintended consequences, as per solutions 1 and 2.

.col-centred {
   margin: 0 auto;
}

<div class="row">
  <div class="col-lg-12">
    <div class="centred">
        Look I am in my own row.
    </div>
  </div>
</div>

Define the width as 960px, or whatever you prefer, and you're good to go!

#main {
 margin: 0 auto !important;
 float: none !important;
 text-align: center;
 width: 960px;
}

(I couldn't figure this out until I fixed the width, nothing else worked.)