Increasing your functional fitness involves training core muscles. You may be strong and lean, but have trouble balancing on one leg. If so, start by adding some one-legged squats to your workouts a few times a week.
When you can handle a one-legged squat with ease, try this move that adds weights: place a small dumbbell on a level bench or chair. Do a one-legged squat and pick up the dumbbell as you come up. Too easy? Place the dumbbell on the floor and pick it up. This move helps train the different muscle groups to work together. Exercising with a fitness ball or wobble board also promotes functional fitness.
But it's important to remember a key difference between functional fitness exercises and standard strength training exercises: Form vs. Fatigue. When you are doing functional exercises, end your set when you can no longer maintain good form. As for standard strength training exercises, you may be working to the point of muscle fatigue.
To add functional fitness and stability exercises into your routine, ask a staff member at your gym to show you some options that are appropriate for your fitness level. Or if you work with a personal trainer and aren't already including stability exercises, talk about adding some. Of course, as when adding any new element to your workout, start easy and work up to tougher moves.