I've noticed that after writing SQL for PostgreSQL, when I view the SQL definition later it seems the database changed the way I formatted the SQL. For example, my leading commas are moved to the back, my tabbing is altered, etc.
Is there a way to prevent this?
Postgres does not "alter the format". The original SQL string is just not stored at all.
The query is parsed and rewritten and depending on the kind of query, action is taken.
For instance, when you create a view, the results are stored in system catalogs, mostly pg_class and pg_rewrite.
All identifiers are resolved at create time, taking the current search_path and visibility into account (early binding). The same query string can mean something different later, if the environment changes meaningfully.
What you see later is a re-engineered version built from these entries. There are some built-in functions to help with that, but it's largely up to the client how to format reverse-engineered SQL code.
Traditional functions are an exception. The function body is saved as string, exactly as passed, after (currently) only superficial syntax testing and validation of objects. If you want a quick way to "save" a query including its format inside Postgres, you can use a PL/pgSQL function, for instance. But see:
Another approach is to maintain an independent code repository like Mike suggested - or not to care about the format too much.