nta786 has written a comprehensive reply, so this is just to add our experiences.
We have found the Premium Membership (£40/35) really useful for buying away tickets. Also living close to London, we tend to go to 5-10 games a year and my teenage son much prefers the away atmosphere. The tickets normally go on sale for premium members at 9:30am and you need to call/go online then to be in with the best chance of getting a choice tickets. This year we went to Chelsea (had very little choice of seats), Watford (good choice but mostly near the front), Norwich (loads of seats available when we booked), Man Utd (reasonable choice) and Brighton (again good choice mostly near the front). Whereas, by the time tickets go on general sale, choice is often very limited. The only games sold out before being offered to premium members which we wanted to see, were Arsenal and today's Brentford game.
As for Plan B, sitting in Itchen North this year, I asked the Newcastle fans sitting next to me how they got tickets and they said they registered with the SFC website!