Does anybody have any tips for performing a wbc diff?

Join the crowd! But I'm glad to see that it is still a required exercise. What if the Coulter counter broke down? You have to learn to be proficient a cell recognition. The pictures in the book are clear cut, but in real life it is sometimes difficult to distinguish between a lymph and a monocyte, especially for beginners. If you find platelets on every field, they're adequate. If you have to really look, they are reduced. Before automated counters we didn't realize that platelets were acute phase reactants.

If you are new at prepping the slide, there can also be problems. Some areas may be too thick, others too thin. You have to look for the Goldilocks zone, then begin your count.

The Wright's stain you got all over your fingers will probably wash off by Easter.

I was taught, on the slide you start where the red cells are just touching each other in a single layer (no significant overlap or piling up). Then you read across the slide, then down and then back again (like a "s") and you should do just a quick scan of the edges to make sure all the platelets are not all clumped there.

There's nothing to doing a differential.  All you're doing is looking at 100 WBCs, identifying what kind they are (Neutrophils, Lymphocytes, Monocytes, Eosinophils, Basophils), and counting how many of each there are in 100 cells.  You use a little counter device that you press down for each type of cell until you get to 100, in which case the device "dings."  Differential done.  Couldn't be easier.  When we used to do hand differentials (before the invention of all these new-fangled machines), the tech assigned to do differentials that day would do 100 diffs before lunch.  Piece of cake.  Skill comes with practice, practice, practice.  Just sit down and start doing differentials every day, just like you learn to read EKG's, and in a few weeks, you'll be a pro.