Today I was entering a bunch of data into a spreadsheet, using the numeric keyboard with my right hand as I read numbers off the list on desk to the left.  In this situation, it's easiest to use the Enter key on the numeric keyboard to move between cells, but when I looked up at the screen, I had entered many columns worth of information into multiple rows of the first column.  For some reason, I though Excel "learned" from your data entry, and would change the behavior of the Enter key based on previous data entry.  So off I go, trying to train it.  Unfortunately, that's not the way it works.  So I resign myself to using the tab key, even though I needed my left hand for moving the ruler on the page.  Several rows of that, and I decide to resort to help.  Office help has gotten so much better that I don't know why I didn't look there first.  I type in "data entry direction" and am briefly discouraged when the first 25 items (of 30) don't answer my question.  But then, there it is, "Change which cell is selected next".  So, that's the story, now here is the tip:

To change which cell is selected next when you press the Enter key in Excel, choose Tools | Options | Edit.  One of the items on this dialog is "Move selection after Enter" and right below it is a dropdown for Direction.  Down was selected, so all I had to do was change it to Right.  Additional tip: if you clear the checkmark in the move selection field, when you hit Enter, the cell selection doesn't change.