Every year my wife and I spend a day throwing out anything we haven't used in a year. Anything that wasn't used for 6 months goes upstairs where it's still accessible, but out of site.

Actually we ebayed a bit last year, and we give stuff to charity shops rather than just throwing it out if we can, but doing this once a year refocuses your view on buying stuff. Will I still use this a year from now has a substantial impact on your buying decisions.

As with many others here, the only expenditure I never regret is travel.