The usual approach is a two-column borderless table.

Suzanne's right; many people do this with a table. You can even set it
up in AutoCorrect or AutoText so you don't have to create the table
every time. A quick newsgroup search should turn up several replies
from the archives that explain how to do that.

Another option is to use MathType, which will do it automatically with
the simple click of a button. You can format the equation numbers
however you want, and you can reference them in the text of your
document. Numbers and references are linked so that if you add or
delete one, everything subsequent to that point in the document is
renumbered. You can do this manually as well, with field codes, but
MathType gives an out-of-the-box solution that doesn't require you to
fiddle with tables or field codes.