If the data source is a table in a Word document, running the following
macro on that document will sort it so that the records are re-ordered to
print down the labels rather than across.  If the data source is not in a
Word document, first use a catalog or directory type mail merge main
document, to get it into that form.  If you don't know what to do with the
macro see the link at the bottom of this message.

' Macro to assign numbers to data source so that it can be sorted to cause
labels to print down columns
Dim Message, Title, Default, labelrows, labelcolumns, i As Integer, j As
Integer, k As Integer
Message = "Enter the number of labels in a row"    ' Set prompt.
Title = "Labels per Row"    ' Set title.
Default = "3"    ' Set default.
' Display message, title, and default value.
labelcolumns = InputBox(Message, Title, Default)
Message = "Enter the number of labels in a column"    ' Set prompt.
Title = "Labels per column"    ' Set title.
Default = "5"    ' Set default.
labelrows = InputBox(Message, Title, Default)
ActiveDocument.Tables(1).Columns.Add
BeforeColumn:=ActiveDocument.Tables(1).Columns(1)
ActiveDocument.Tables(1).Rows(1).Range.Cut
k = 1
For i = 1 To ActiveDocument.Tables(1).Rows.Count - labelcolumns
   For j = 1 To labelrows
       ActiveDocument.Tables(1).Cell(i, 1).Range.InsertBefore k + (j - 1) *
labelcolumns
       i = i + 1
   Next j
   k = k + 1
   i = i - 1
   If k Mod labelcolumns = 1 Then k = k - labelcolumns + labelcolumns *
labelrows
Next i
ActiveDocument.Tables(1).Sort FieldNumber:="Column 1"
ActiveDocument.Tables(1).Rows(1).Select
Selection.Paste
ActiveDocument.Tables(1).Columns(1).Delete

See the article "What do I do with macros sent to me by other newsgroup
readers to help me out?" at:

http://www.word.mvps.org/FAQs/MacrosVBA/CreateAMacro.htm