well gaining weight is about putting more calories in then your using. So if you want to keep the amount you put in the same you must increase your expenditure aswell. However there is little point in using them up by performing lots of aerobic cardio work if all you want is to become big right now. So i assume what Fadi means is to increase calorie usage in a way that will help your goals- not just going through the motions.
having said that there are plenty of ways in which you can increase your calorie use in training- if your not already lifting heavy then that is one way - more training sessions are a possible way although how many do you do already? you could also slightly increase the volume (add a couple of sets) of your current program...
However if you feel as though your current program is giving you good results other than the extra fat i personally would just decrease my calorie intake a little. This is because its quite simple for most people to eat an extra 500 calories a day and a lot harder to use an extra 500 on top of the training they are already doing - as long as what you are currently doing is optimal.