Have a look at Micrel's MIC4680. http://www.micrel.com/page.do?page=/product-info/products/mic4680.shtml
Download the datasheet. Have a look at the 1st schematic on the right. If you replaced R1/R2 with a 5K pot, you would have an adjustable "buck" DC-DC converter with a minimum of components (MIC4680 + 5 external components), capable of up to 1A output - and it would be far more efficient than a linear regulator.