Are plastic bags good for anything? They are an oil product, so use up valuable resources. They are a major constituent of landfill sites and don't degrade, clogging up the earth. They are discarded and end up caught in trees and bushes (hence witches' knickers), defacing the environment.
San Francisco has now introduced a city-wide ban, joining South Africa (where you could be sent to jail if found guilty of distributing plastic bags), Rwanda, Bhutan, Mumbai and Bangladesh where they cause another environmental problem: drains blocked by discarded bags cause flooding. By the end of the year Paris will also have banned them, the whole of France by 2010. In America lobbyists for the bags (who would have thought there could be such people?) fought against San Francisco's decision because they feared it would kick-start a nationwide trend: which has indeed happened now other cities such as New York and LA are investigating bans or taxes.
So what can we do to reduce the use of plastic bags? Most shops will give you a bag as a matter of course: make a point of taking your own reusable bag and saying "no thanks, I have a bag" or "no thanks, I don't need one". If you can fit your shopping into your handbag, do that, if not, carry a bigger fabric or string bag folded up in your bag or pocket. It won't take up much space and will allow you to refuse a plastic one.
If you must get a plastic bag, then reuse or recycle the ones you accumalate at home. Using them as bin liners is the classic reuse scenario, but as they will end up in landfill, it may be better to buy compostable sacks and recycle the bags. UK supermarkets now generally have recycle bins for plastic bags, and some local authorities will take them in the weekly recycling collections (check on your local council's website as to what plastics you can put in your green bin).
Banning plastic bags is not the end of the problem though - packaging takes up a far greater percentage of landfill: Should we leave packaging at the checkout?