Ingrid,
another option is to rent the lens you're thinking of buying either instead of buying it or as a test-drive prior to the purchase.

A new lens rental site has opened up on the web -- www.rentglass.com -- and they are currently offering rentals on Canon L glass and will soon be offering Nikon glass as well. The rental rates are pretty reasonable.

This sounds like a reasonable alternative if you have a need for a lens for a particular event -- like a 300mm F2.8 -- but don't want to invest in ownership of that particular lens.

Jim