There are many free worksheets available, especially online, but still the best worksheet is one that you personally draft. This way, you are able to match the level of difficulty of the activity in accordance to the performance level of your own students. It is not bad to reuse worksheets for another batch of students, but once in a while it is also better to vary the activities you give to kids. Worksheets can be made for fun if it is attuned to the current interests of kids. The kids will respond better to activities close to their own interests.
Mathematics skills can improve with just one technique -practice! Most of the children end up petrified of numbers and number games because of lack of appropriate knowledge. All this is a result of poor teaching mechanics. The vast school curriculum does not teach the children the perfect way of learning the subject. It is only proper practice and regularity that can make a student reach perfection.