This technique will let you show a rule of thirds grid, no matter what size or resolution of your image. Can't actually remember where I came across this tip, but I find it very useful.
First open the Preferences... Guides, Grid, Slices from the Edit menu
Alter the setting at shown in the image below.
Click OK, then from the View menu, click on Show... Grid
Your grid should now appear over your image. Best thing about this is that when you resize your image either in pixels size or resolution, the grid will automatically adjust itself to your new settings.
Hope this is of some help to anyone.