I would suggest that between inclusion in sports that are already an activity within the family., or school. Exposure to other sports that are prevalent and prove to be enjoyable to the child.
Education at an early age of the advantages and variety of sports that are available, and present role models of those different sports to the child so that they appreciate the dedication required and the goals and accolade that can be achieved.
All too many times I have seen parents push their children in a direction of a sport that appeals to themselves, or was a sport they were good at when younger. The problem with that is there are so many variations, and kids have often very different ideas than their parents. How many great sports men and women could have there been only if they had been introduced to a sport they really loved and took to at the right time.. Of course with so many options it can be difficult for parents, or pre school and primary schools, to find the right key for each child. I don't doubt that each child has a particular bias to a type of sport, although that doesn't necessarily mean they will excel at it, for a number of reasons.
Importantly there are two factors here, in the first place they should whatever be allowed the opportunity, and secondly if they do wane and really cease to be serious, they should'nt be pushed into it further, because of a parents wishes, or as I hear often, the parents have spent a lot of money on this sport, so the child must do it. It would be more constructive to assess the positives from the sport that has proved to be unsuccessful, and offer the child alternatives.
It can be costly, if for each sport tried the child is bought every conceviable item concerned with the sport they choose to try. But it doesn't have to be that way. For many sports there are outlets and clubs where by using the venues equipment a "try it and see" can be arranged. If not, many sports can be tried with either cheaper second hand equipment, or through buying the basic requirements of the sport, or even borrowing a friends and joining in. Perhaps these points covered are the main reasons why there are not many more sports achievers, because the overseers whether it be the parents or schools choose not to promote sport as a worthwhile activity because of cost, available facilities, or simply the inteference of all the other facets that encroach on modern day living.
In conclusion, education and awareness is primary to ignite that spark in the children, but in addition, and indeed more importantly, there needs to be an education of the people who are in a position to guide and influence the children in the first place, and continue to maintain a realistic level of encouragement.
All mine, and having two boys of my own with sports a orientation.