Conditionally altering tables

Coming from Microsoft SQL Server here...

Is there a way to write a script so that it will make required schema
changes, only if they haven't already been done?

Eg, I have a script that should add a NewField field to MyTable, as so:

ALTER TABLE MyTable ADD COLUMN NewField integer NULL

This works fine the first time it runs, but if I run this script against
the database again, I get a message saying that NewField already exists,
and the script errors out. In MSSQL, I could write something like:

IF NOT EXISTS (SELECT * FROM sysobjects so INNER JOIN syscolumns sc ON
so.id = sc.id WHERE so.Name = 'MyTable' and sc.Name = 'NewField) ALTER
TABLE MyTable ADD COLUMN NewField integer NULL

And it would be fine. Is there something similar I could do in Postgres
without having to write individual stored procedures for each one?

--Colin McGuigan