I think that if you want to achieve your goals, you've got to be serious about it. Unfortunately, the word "serious" brings with it connotations of solemnity, toughness, frowns...
I think that it shouldn't be that way. Being serious means that you really want something. It means that it is a high priority. In fact, I think that if you are serious, you can actually have fun, it means that the goal resonates with your entire being, and that you are joyful in the working out of your goal with passion. It means that you don't have to cater to every whim and fancy that comes in your way or be burdened by 1000 other trivial things. It means you can say "No" with certainty, be firm, and not feel guilty about saying no.
There are people who say they want to lose weight but aren't serious about it. They buy slimming teas or drinks, yet eat a candy bar the next hour.
Any goal that is worthwhile to pursue should be pursued with seriousness.