Ideally the simplest way of reducing lighting costs with downlights would be to replace the inefficient halogen globe with an efficient compact fluorescent (CFL) or LED globe (although the problems with heat loss and multitude of fittings still remain).
Downligths come as 12 or 240 volt, the type must first be identified (majority of downligths are 12 volt). Here is a guide to determine which voltage you have.
The simplest option is to replace the standard 50watt dichroic MR 16 globe with a more efficient halogen globe such as an IRC globe (eg. 20 watt Phillips ES - LV halogen IRC halogen globe). These globes can immediately replace any halogen globe and reduce lighting energy requrirements plus reduce fire risk as they produce much less heat than standard halogen globes (light is equivalent to a 35 watt standard halogen). They go some way to reducing lighting costs, but still retain problems of air leakage and number of fittings etc.
12 Volt fittings
The tyype of transformer determines whether you can use a direct globe repalcement. To determine the type of transformer you have
Energy efficient globes
adaptor is fitted to the T5 globes.