All the tips and tricks above are great. Pick and chose which ones work for you, and climb more hills. You don't get better without pushing yourself.
I ride singlespeed bikes, so I don't get a choice in gears. There's sit and mash, stand and hammer, or get off and walk. I've used a mix of all 3 on some courses.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.