The bottom line is, portable air conditioners and window air conditioners are not as efficient as central air conditioning systems. They have to work harder and create more cooling power to achieve the same effect.
However, some models are far more effective and efficient compared to other models. Make sure yours is an Energy Star air conditioning unit. In regard to window AC units, buy one that's sized correctly to your space - don't over or under cool - and, again, that's Energy Star rated.
When it comes to making your home receptive to the cooled air, general home efficiency rules apply. If you don't have attic insulation, or don't have enough of it, your house will be hotter in the summer and cooler in winter, costing you big. Make some basic efficiency improvements to your home and you'll feel a big difference.