Generally shields are put up right before a big hit. So if a tank buster is coming you would cast a shield in your tank before they are hit. HoTs are good to keep up on your tank at all times, and AoE HoTs are most efficient when you know a lot of AoE is coming. HoTs can function as a nice safety net for DPS as they will be healed overtime and you won't have to spend MP or time on healing them unless they desperately need it.

As a side note: Shield before pulls = good. HoTs before pulls = bad.