Appointments are necessary and should be made one month in advance.
It is good idea to send a list of who will be attending the meeting and their credentials so that Thais know the relative status of the people attending the meeting and can plan properly.
You should arrive at meetings on time as it signifies respect for the person you are meeting.
Although most Thais will try to be on time, punctuality is a personal trait.
Always send an agenda and material about your company as well as data to substantiate your position prior to the meeting. Allow sufficient time for the material to be reviewed and digested.
Remain standing until told where to sit. The hierarchical culture has strict rules about rank and position in the group.
Written material should be available in both English and Thai.
You must be patient.