Agree that being too close can be a problem. I did this at first with my macro(Sigma 50mm on D70 Nikon).  

To test depth of field and focus, I set up toothpicks in a row going from me and marked one with a red color. Then, I was able to judge the dof with various aperture from focus of the toothpicks. Sure, this was simply an experiment but told me more than I really wanted to know about dof!

Manual focus was used since auto would not take one close toothpick over another and made "its own choice".    A problem is getting high number f stop and having enough light while at a decent shutter speed.  Tripod is the way to go for non-changing or non-moving subjects. 

Also, sometimes try the focus somewhere inbetween the limits of the dof.   Just my 1/2 cent worth.

TomDart.