You didn't say if you were connecting to Phuket, or spending a few days in BKK before going to Phuket?

The interline tip mentioned in the last post was good. But also note the following:

My last trip into Thailand, I had three checked bags and stopped at BKK for two days before proceeding on to a domestic city on THAI. I left my baggage tags (from the USA) on the bags and at THAI check in, the agent was clearly concerned about the number of bags and weight of them. Without even saying anything though, after looking at the bag tags she proceeded on with the check in, effectively maintaining the "piece concept" granted on flights from the USA.

This would be expected on an actual connection. On a two day stopover in BKK however, it is much appreciated. I know airlines in my own country would be trying to make me cough up some cash in the same situation.