How can I prevent post support brackets from creaking under foot traffic?
Like Ed Beal suggested in a comment, you mostly need to drive your shims in more assertively. However, I'd use plenty of construction adhesive as well. 


Remove the shims now in place if they'll come out with a little effort. 
Blow or vacuum out all dust and debris. 
Inject plenty of heavy-duty construction adhesive into all gaps. 
Firmly drive pressure-treated wood or synthetic shims into any gap, above or below the metal brackets. 
Add more adhesive around the shims to lock them in place. 


24 hours later everything should be really well stabilized and silent.