To my mind, the most important behavioral changes are: 1. include movement in their daily routine; 2. get adequate rest; 3. reduce stress as much as possible; and 4. be mindful of portion sizes.
It will depend on a few different factors, such as what stage of change your client is in. If they are ready to make a change, then here are a few strategies:
1. Setting small but attainable goals is important for success. Having the client be accountable for things such as recording daily food intake or even showing up for an exercise session could be a good start.
2. A supportive environment from family and friends plays a big role in success and helps motivation.
3. Have a plan in place to overcome barriers to eating well or exercising. Examples are to plan healthy, low-fat meals ahead of time and have clothes ready to go for the gym.
4. Guidelines for proper nutrition, as well as a set exercise program that includes both cardio and strength training will help clients achieve goals of fat loss.
Hope this helps.
In my opinion support from family and friends is the most important part. Anyone who is trying to lose weight a supportive environment around him/her will help to overcome any obstacles (mostly mental) that might arise. The right diet plan along with an effective fitness program are the other two factors that will help this person to reach the goal.