Your trinkets are in slots 13 and 14. So if I wanted to blow my trinkets every time before using Shield of Righteousness, I would make a macro as follows:
#showtooltip Shield of Righteousness
/use 13
/use 14
/cast Shield of Righteousness
If you only want to use one trinket slot as your "every time" and save the other for your "ohcrap", then just use one of them.
Adding an extra 400 block value just before a shield slam can be a bit fun
However, I don't have a threat issue so I am still using Commendation of Kael'Thas for the extra 57 stam and automatic "ohcrap" button, and I can't justify swapping that our for a trinket that gives passive block rating that would put me well over block cap.