One of the easiest ways to do this is to install an Expresso Bar/or machine on the lot you plan to visit. When the energy starts getting down low, just have them drink some expresso and it will raise their energy slightly. The more you drink the more they will gain. Be warned tho that it does affect their bladder need almost in the exact opposite of the energy level boost.
There are also several different mods available for keeping their needs fully maxed. I'm sure you'll get a couple of links to those.
