When starting a new project in Axure you should start with a grid so that your layouts stay consistent from page to page. (An updated version of this post is available here.) I recommend using the 960 Grid System as the basis of your Master page layouts. This will ensure that you are designing your wireframes at a size that is optimized for 1024×768 browsers and that your page templates will be compatible with good responsive design practices.
In Axure, if you select Wireframe > Grid and Guides > Create Guides in the main menu (or right click on the wireframe grid and pick from the context menu), it opens the Create Guides dialog. In that dialog you can select the 960 Grid 12 Column or 16 Column guides. However, this gives you the guides for all 12 or 16 columns, which in my opinion clutters up the screen. Of course one way to deal with that is just remove the guides you don’t want.
Another way is to only set the guides you want in the first place. I find it easier to visualize a page layout by placing guides on top of a 960 Grid image. To do this, open a new Axure project and place either the 12 Column Grid image (download the 12 Column image here) or the 16 Column Grid image (download the 16 Column image here) on a Master page. To create global guides for all the pages, hold Cmd (Mac) or Ctrl (PC) and drag a guide from the ruler until it is positioned where you want it on top the grid image. You can then just delete the image, or duplicate the Master page, delete the image on the duplicate, and start building your Masters.
You can download an Axure file with the 960 Grids already placed on Master pages (shown below — click for larger view). Enjoy!