I rarely have issues getting them on United or Delta/NW.

United requires you to be a 50,000 mile/60 segment flyer called Premier Executive.

I believe with Delta/NW it kicks in at 25,000 mile/30 segments called Silver.

If you do not have elite status, then you have to wait until the 24 hour window and hope they are not all taken by elites.

With United you can always just pay for access to Economy Plus and get ~5 more inches. That to me is better than an emergency row b/c they don't get as cold.