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Depends on why you are thanking them. doumo arigatou gozaimasu means "thank you very much" and can be used towards anyone, it might sound fake if used in a situation where you really aren't all that thankful though, for example to a clerk at a seven eleven.

どうもありがとうございました !
Say that while bowing quite low a couple of times.
Oh and you can repeat ありがとうございました a couple of times.

That's pretty much the best you can do.
Need to be in the right context though.