As you climb - keep dropping to lower gears to keep your comfortable spin rate, until you run out! Keep a lookout fo rspots where you can stop and take a breather if you wish - somewhere to put your left foot away from roadside nettles, a shady spot on a hot sunny day, that sort of thing. And take the break if you feel it necessary! Keep up the steady breathing - but don't try and keep up your road speed, just the comfortable pedal spin speed.
Don't get off and push unless it really is too steep to pedal and/or you have a heavy load! (I've done this on a bend on a 1 in 4 with loaded panniers, (up the hill into Harlech), when I simply toppled over going so slowly)!
As you reach the top and the pedalling gets easier - delay each change up of gear, and keep up the same pedal rate, for a few seconds longer than you think necessary, this will give your legs/breathing etc time to recover.
Above all - take time to watch the scenery go by and enjoy going down the other side!
If you regularly run out of low gears far to early, consider modifying the bike. For background, mine are 20" - 110" and the 20" gets used a lot! The Brompton of course doesn't go that low so I push - oops
PS - a technique I used to use until quite recently on very quiet roads was to zigzag up the hill, but its getting less and less possible these days since the advent of satnav (think about it!)
