Prakash has a very good post on Maintainability here. Please go read it first.
I will expand on pair programming point a bit and add my perspective.
You add more members to the team. They start writing code. Even though you can educate your developers about how the code is written, but everyone brings some baggage with them (culture, how they worked in previous projects, companies etc…)
There is a gap in terms of skill level, experience level and understanding (being on the same page with respect to the architecture, design, development practices) when the team ramps up. We need to pair at least till the gap is bridged.
I see two aspects to maintainability, preventive and reactive. Both are equally important. Pairing is probably one of the tools in the the former and reviews the later. Reactive probably has a negative connotation to it so i might reword it as ongoing effort or continuous effort.
This just like how we should handle Performance. Design for performance and still plan for performance tuning when things fall out of place.
To generalize aim for the best and ideal, you won’t reach there because there is always a gap between ideal and real. So when things fall out of place have checks and counter measures to pull them back on track.
As some one who hates reviews I have come to a compromise that they are necessary evil.
Srini has a great article on the topics mentioned above. It is definitely worth a read.
Ideas Are Worthless…
The value of ideas laid in their execution
idea’s relative lack of value in the absence of unyielding execution.
from http://infochachkie.com/spilling-the-beans/ found via http://www.quora.com/What-should-the-Winklevoss-twins-have-done-to-protect-their-idea
People waste tremendous amounts of emotional energy on things that can’t be changed — notably other people and their behavior. Does it really do any good to get angry at that jackass driver who cut you off on the freeway? It won’t change anything. Even if you yell or honk he probably won’t notice or care. The anger makes you feel worse, not better. The same can be said for holding grudges or harboring resentments — it’s "like drinking poison and expecting someone else to die."