Easy to implement tip!
Include demanding exercises in your gym routine. The most challenging types of exercises involve free weights and different forms of body weight exercises. These exercises require more effort and calorie expenditure than seated exercises on a machine. They also keep the metabolism running higher for longer after your workout, meaning that you burn more calories when you are resting.

Some examples:
Squatting
Lunging
Pulling
Pushing
Bending
Twisting

Equipment you can use:
Your own body weight
Dumbbells
Barbells
Kettlebells
Cable machines
Medicine balls
Swiss balls
Chin up/dip bars
TRX
Sandbags