can't fight your way out of a paper bag    

If you say that someone can't fight their way out of a paper bag, you are saying in a contemptuous way that they are very bad at fighting.

We've already shown you that are no use to you as allies. They couldn't fight their way out of a paper bag.

You can replace `fight' with other verbs that state what someone is incapable of doing.

Certainly, too, the democratic parties that support Mr Yeltsin could not organise their way out of a paper bag.

not worth the paper it's written on

If you say that a promise, agreement, or guarantee is not worth the paper it's written on, you mean that although it appears to be official or definite, it is in fact worthless. `Printed' can be used instead of `written'.

If consumers are unaware of their right to compensation when a service breaks down, guaranteed service standards will not be worth the paper they are written on.

The certificate is not worth the paper it is printed on.

on paper 

If you say that something looks or sounds good on paper, you mean that it seems to be a good idea, plan, or argument when you read or hear about it, but may not be good in reality.

This system looks good on paper but it is expensive and, in my view, still of very limited value.

These reforms are more impressive on paper than in reality.

a paper tiger

If you say that a person, country, or organization is a paper tiger, you mean that although they seem to be powerful, they do not really have any power.

The Khmer does not succeed in derailing the polls, has already managed to make the U.N. in Cambodia look like an expensive paper tiger, without the teeth to carry out what it was sent to do.

Russia's Asian forces are a paper tiger these days. Starved of fuel and spare parts, low on morale, they are barely in shape even for manoeuvres, let alone war.

a paper trail

Written evidence of someone's activities can be referred to as a paper trail. This expression is used mainly in American English.

The criminal proceedings were raised after investigations found a paper trail of checks that were written on dummy bank accounts.