6) Never stand up*(see note)
*The only notable exceptions to this rule are i) stretching, ii) breaking away and iii) stopping falling off.
i) In this country, you shouldn’t need to stand to stretch on climbs at all, this is only really intended for climbs that are 2 or more miles long. Alpine ascents are a good example of when to do this.
iii) This is reserved for really steep climbs or moderate climbs with luggage. With a triple you should be able to manage anything (I will excuse people who go up Church Hill in Wales – 33%, not a typo and yes it can be done!), with a double, gradients of above 15% are worthy causes. I have been up a 20%er sitting down though, so it can be done.
If you are doing a particularly long hill, you can shift position to accentuate different muscle groups. Moving forwards in the saddle and pointing the toes down uses the calf muscles more, moving back in the saddle and cycling with the heel pointing slightly down (or a level foot) will use the quads more. Mentally you can also pick a point to reach that is achievable, get there and pick another point. Don't focus on the whole hill, just bits at a time. Makes it a lot easier psychologically.
Gonzo, you live somewhere flat!
Even my commute from the Vale of York to Leeds includes a 2km continuous climb (albeit a very gentle gradient).
There are loads of climbs in Yorkshire over a mile long.
On longer climbs, switching between sitting down and standing relieves some muscles. It also relieves the tedium of a long slow slog.
If not racing, I've found that gentle riding out of the saddle can be easier (ie, on the heart and lungs) that sitting down for steep gradients (over 20%). I think it is partly because it is easier to ride very slowly standing up.
For a long slog of a climb, try this one
http://www.gmap-pedometer.com/?r=17782776.7miles of continuous climbing, no letup. 1100ft of climbing. Not steep, just relentless. That's a climb for sitting down and spinning. And patience.
(I used to ride it every morning, on the way to the train. In winter, with a headwind, it reduced me to a weeping wreck)