Horizontal list items

So, I have attempted to create a horizontal list for use on a new website I am designing. I have attempted a number of the suggestions found online already such as setting 'float' to left and such - yet none of these have worked when it comes to fixing the problem.

    ul#menuItems {
      background: none;
      height: 50px;
      width: 100px;
      margin: 0;
      padding: 0;
    }
    ul#menuItems li {
      display: inline;
      list-style: none;
      margin-left: auto;
      margin-right: auto;
      top: 0px;
      height: 50px;
    }
    ul#menuItems li a {
      font-family: Arial, Helvetica, sans-serif;
      text-decoration: none;
      font-weight: bolder;
      color: #000;
      height: 50px;
      width: auto;
      display: block;
      text-align: center;
      line-height: 50px;
    }

<ul id="menuItems">
  <li>
    <a href="index.php">Home</a>
  </li>
  <li>
    <a href="index.php">DJ Profiles</a>
  </li>
</ul>

Currently I am unsure of what is causing this issue, how would I go about and resolve it?

Updated Answer

I've noticed a lot of people are using this answer so I decided to update it a little bit. No longer including support for now-unsupported browsers.

ul > li {
    display: inline-block;
    /* You can also add some margins here to make it look prettier */
}

<ul>
    <li> <a href="#">some item</a>

    </li>
    <li> <a href="#">another item</a>

    </li>
</ul>

I guess the simple solution i found is below

ul{
    display:flex;
}

This fiddle shows how

http://jsfiddle.net/9th7X/

ul, li {
    display:inline
}

Great references on lists and css here:

http://alistapart.com/article/taminglists/

A much better way is to use inline-block, because you don't need to use clear:both at the end of your list anymore.

Try this:

<ul>
    <li>
        <a href="#">some item</a>
    </li>
    <li>
        <a href="#">another item</a>
    </li>
</ul>

CSS:

ul > li{
    display:inline-block;
}

Have a look at it here : http://jsfiddle.net/shahverdy/4N6Ap/

You could also use inline blocks to avoid floating elements

<ul>
    <li>
        <a href="#">some item</a>
    </li>
    <li>
        <a href="#">another item</a>
   </li>
</ul>

and then style as:

li{
    /* with fix for IE */
    display:inline;
    display:inline-block;
    zoom:1;
    /*
    additional styles to make it look nice
    */
 }

that way you wont need to float anything, eliminating the need for clearfixes

Here you can find a working example, with some more suggestions about dynamic resizing of the list.

I've used display:inline-block and a percentage padding so that the parent list can dynamically change size:

display:inline-block;
padding:10px 1%;
width: 30%

plus two more rules to remove padding for the first and last items.

ul#menuItems li:first-child{padding-left:0;}
ul#menuItems li:last-child{padding-right:0;}