how do you make the star effect on a cake look neat and tidy and not messy??

To do a star tip fill in, you want to use medium consistency icing and work your design row by row. Try this link to show you the correct way to do it
http://www.wilton.com/decorating/basic/stars.cfm
You may want to check this out. It has all kinds of information on icing consistency, filling bags, and links for other decorating how to's. http://www.wilton.com/decorating/basic/index.cfm
This topic discussed a practice icing and how to make it to correct consistency and you can apply the same directions to Wilton's buttercream by using milk instead of water. http://www.cakecentral.com/cake-decorating-ftopict-9955-class.html+buttercream. I hope this helps!

Another way to make it look neat is to make it look uniform as suggested row by row and take your time. Before pulling up stop your flow to avoid tips at the top of the star. There is a tip that allow you to do three at a time. I really don't like to use it but others say it saves time.