How to change gear ratios for hill climbing?

I have a Compact ISIS Crankset 50/36 with a Ultegra 6600 10spd 11-23 cassette and I need slightly lower gearing for better hill climbing on my roadbike. What is the recommended way?How to change gear ratios for hill climbing?
Buy a 11-25 or 11-27 cassette. You can go with a 12 small cog if you like as well.How to change gear ratios for hill climbing?
Shimano makes a 12-27 cassette that should work. With a road bike, you can't go crazy big on the cassette (29 tooth gear) since the rear derailleur won't be able to take able to take up the difference in chain. Also, going with a larger cassette might cause you to have to get a chain that is a couple of links longer.



You need to remove the quick release from the rear wheel. There is a special tool to remove the cassette from a Shimano equipped wheel. You can get a Spin doctor or Park one, here is an example (not endorsing this specific one but I am sure it is fine.)

http://www.performancebike.com/shop/prof

You also need a chain whip.

http://www.performancebike.com/shop/prof



You need to remove the lockring turning it counter clockwise and the chain whip to hold the cassette. Pull the gears straight off and slide the new cassette on. Be careful to keep all the spacers and gears in the same order that they came. The hub is keyed so you can only orient the cassette in one direction. You need to use a bit of force to tighten the lockring on the new cassette pulling in teh opposite direction with the chain whip.
To add to MR's post if 11-25 or 11-27 is not enough, Campy has a 13-29.



EDIT: rear cassette size limit is due to the derailleurs cage length, if you have a long cage this is not an issue and adding a few extra links is no big deal.
we did a very hilly tour this weekend. i run a 39t inner ring and a 27t in the rear, but there was one hill i couldnt make. i suspect that if you change to a 27t rear cluster you should have pleanty, and they make a 34t inner ring for compact cranks.