If you can lift a weight for more than 15 reps then it’s too light. This will build muscular endurance but not strength, Donavanik says. “You want something that’s going to push and challenge you.”
There are lots of ways to format your workout, but Donavanik suggests increasing the weight a bit while decreasing reps with each set — starting, for example, with a weight that you can lift relatively easily for about 10 to 12 reps. “It’s a good pump to get the muscles going, to get blood-flow, to get warmed up," he says. Follow that up with eight to ten reps with a weight that's a bit heavier, then six to eight reps with a weight a little heavier than that, and so on. While the amount by which you up the weight each time will depend on your own body and abilities, Donavanik says that the final one to three reps should always be challenging.
If you’re somewhat new to working out, Donavanik's format will give you an idea of where you max out and what your limits are. After a month or two of getting to know your limits, you can move on to other formats, like four sets of eight reps, or five sets of six reps. You’ll know it’s time to increase the weight once the last couple of sets become easier. “There’s a lot of variation with reps and sets, not one strict regimen,” he says.