I'd actually say that you need to get out of the saddle sooner (depending upon the length of the 20% part). If you leave it until the last second you will be grinding and exhaust yourself before you get to the serious climbing. On the lead up climb, regularly standup, even if just for a few seconds, to allow yourself to stretch out and relax. Keep your cadence nice and high and spin. When the going gets tough, stand up smoothly and keep the rpm in the 70-80's if possible. Over-revving whilst standing will tire you very quickly. Remember to breathe in time with your pedal strokes to keep the O2 high, try not to gasp. If you ride clipless then you can also employ the "pull" technique, whereby you can pull up on the pedals whilst standing too. This can produce MUCH more power but at the expense of energy, so use sparingly.
As others have said the only real way to cure the hill "blues" is practice, practice, practice.