Looking at some of the earlier posts they appear to be talking about storing the paint tins inverted. I was taught to invert the tin (after ensuring the lid is well sealed - not a mistake you'll make twice...), swirl it round briefly, and then store upright. The idea being that the film of paint from the inversion provides an airtight seal. That said I've normally ended up moving house before needing to use the long since forgotten paint, so can't really speak for the results past a year or so.
The advice from the nice shaggy dog at Dulux (http://www.dulux.co.uk/help-advice/environment/) is:
The advice from the nice shaggy dog at Dulux (http://www.dulux.co.uk/help-advice/environment/) is:
Reduce the paint you buy Reduce the amount of paint you have left over by calculating exactly how much you need for your decorating. At Dulux we have created a handy paint calculator to do this for you, but if you are in store then you need to look at the back of the paint container and check the coverage per litre that the paint gives you.
Leftover paint will last longer if the tin is sealed tightly. To reseal a can of paint so that it is airtight, wipe the rim of the can clean, replace the lid, place a block of wood over the top and tap it down with a hammer. Then quickly invert the securely lidded can to form an airtight seal which will help prevent skinning.
Store leftover paint upright in a frost free environment, and keep out of sight and reach of children.
If you have any usable surplus paint, offer it to friends, local community groups or you can contact Community RePaint.
Community RePaint provides a practical solution to this waste stream that is environmentally and socially beneficial. Unwanted surplus paint is diverted from the waste stream and redistributed for re-use by community groups, charities and voluntary groups. Co-ordinated by the not-for-profit environmental consultancy Resource Futures, Community RePaint schemes collect 'half-tins' of leftover paint from donation points at some Local Authorities’ Household Waste Recycling Centres or Civic Amenity sites.
You may be able to donate paint through your Local Authority Household Waste Recycling Centre or visit http://www.communityrepaint.org.uk where you can enter your postcode and find your nearest Paint Donation Facility.
For responsible disposal of other waste paint call your Local Authority for guidance, as some have special facilities for disposing of paint.
Do not pour paint down drains or watercourses.
Empty paint containers can be recycled. Ask your Local Authority how this should be done as many do not want waste paint containers put into dustbins. If your containers can’t be recycled locally then dispose of them responsibly as general household waste.