How do I get the current unix timestamp from PostgreSQL?

In postgres, timestamp with time zone can be abbreviated as timestamptz, and timestamp without time zone as timestamp. I will use the shorter type names for simplicity.

Getting the Unix timestamp from a postgres timestamptz like now() is simple, as you say, just:

select extract(epoch from now());

That's really all you need to know about getting the absolute time from anything of type timestamptz, including now().

Things only get complicated when you have a timestamp field.

When you put timestamptz data like now() into that field, it will first be converted to a particular timezone (either explicitly with at time zone or by converting to the session timezone) and the timezone information is discarded. It no longer refers to an absolute time. This is why you don't usually want to store timestamps as timestamp and would normally use timestamptz — maybe a film gets released at 6pm on a particular date in every timezone, that's the kind of use case.

If you only ever work in a single time zone you might get away with (mis)using timestamp. Conversion back to timestamptz is clever enough to cope with DST, and the timestamps are assumed, for conversion purposes, to be in the current time zone. Here's an example for GMT/BST:

select '2011-03-27 00:59:00.0+00'::timestamptz::timestamp::timestamptz
     , '2011-03-27 01:00:00.0+00'::timestamptz::timestamp::timestamptz;

/*
|timestamptz           |timestamptz           |
|:---------------------|:---------------------|
|2011-03-27 00:59:00+00|2011-03-27 02:00:00+01|
*/

_DBFiddle

But, note the following confusing behaviour:

set timezone to 0;

values(1, '1970-01-01 00:00:00+00'::timestamp::timestamptz)
    , (2, '1970-01-01 00:00:00+02'::timestamp::timestamptz);

/*
|column1|column2               |
|------:|:---------------------|
|      1|1970-01-01 00:00:00+00|
|      2|1970-01-01 00:00:00+00|
*/

_DBFiddle

This is because:

PostgreSQL never examines the content of a literal string before determining its type, and therefore will treat both […] as timestamp without time zone. To ensure that a literal is treated as timestamp with time zone, give it the correct explicit type…In a literal that has been determined to be timestamp without time zone, PostgreSQL will silently ignore any time zone indication

SELECT extract(epoch from now() at time zone 'utc');

doesn't return the correct timestamp because postgres timezone conversion throws away timezone information from the result:

9.9.3. AT TIME ZONE

Syntax: timestamp without time zone AT TIME ZONE zone
Returns: timestamp with time zone
Treat given time stamp without time zone as located in the specified time zone

Syntax: timestamp with time zone AT TIME ZONE zone
Returns: timestamp without time zone
Convert given time stamp with time zone to the new time zone, with no time zone designation

afterwards, extract looks at timestamp without time zone and considers it to be a local time (although it is already utc in fact).

The correct way would be:

select now(),
       extract(epoch from now()),                                          -- correct
       extract(epoch from now() at time zone 'utc'),                       -- incorrect
       extract(epoch from now() at time zone 'utc' at time zone 'utc');    -- correct

          now                  |    date_part     |    date_part     |    date_part
-------------------------------+------------------+------------------+------------------
 2014-10-14 10:19:23.726908+02 | 1413274763.72691 | 1413267563.72691 | 1413274763.72691
(1 row)

In the last line the first at time zone performs the conversion, the second one assigns new time zone to the result.