@Richard *******
For the 90days, there is nothing to do.
The 90days starts again from 0, the day you come back in the country. It is automatic, no need to go anywhere.
I just went to Vietnam few week ago, and didn't di anything for 90Days or TM30 (but i'm "manage" by Chiang Mai Immigration, and could be quite different in another part of Thailand : each Immigration bureau wrks a bit as it's main officer decides).