As others have said pacing yourself is the biggest challenge. If you go too hard at the start you'll blow up, if you spin up in your granny ring you'll never get faster. I immediately improved my hill climbing times when I started approaching the hill with good speed, and starting off the hill sitting down in a relatively easy gear. I like to alternate between sitting and standing when I climb, maybe 65:35 in favour of sitting, so when I first stand up I'll shift up 2 or 3 and then gear down 1 or 2 when I sit down. After a couple cycles of this I can usually find comfortable but challenging gear.

When I first started I would mash pretty badly when I stood up, and only for a short time before my legs were shot, but with a little practice you can build a really good technique for dancing up the hill. Then again I find my lungs are usually the most painful when I attack hills so maybe that's why I favour going anaerobic more frequently.