If you stick to the same foods all the time the chances are that you aren't getting enough of the wide range of nutrients you need. Particularly if you don't eat many fruit and vegetables.
Always have museli for breakfast? add a different topping every day for example crushed nuts or seeds, chopped dried figs or dates, a handful of berries or chopped banana.
That plain chicken sandwich you have for lunch - add loads of salad, try different tpes of bread or ditch the bread and have a big salad with the chicken.
To cassaroles or curries add a tin of beans or chickpeas, a big handful or spinach or mushrooms
Think of a rainbow and try and make sure that you eat fruit and vegetables from each colour group every day.