Fold the rim tape and cut a diamond to provide generous clearance for the valve before applying the tape. Also, use a bit more contact adhesive to seal down the overlap when you finish. Good practice when fitting inner tubes is to use plenty of talcum powder. Not only will this neutralise any adhesive over-spill, but it will remove the stickiness from the rubber. This will make the tube easier to handle, and will allow it to move independently of the inside of the tyre, reducing stress during inflation and aiding suppleness when riding. The best racing inner tubes are supplied "pre-talced" for these reasons.