Wash all instruments with alchohol or Bleach between samples. Be certain there is no excess water on the leaves prior to drying. You can also cut leaf pieces with scissors, there is some debate on whether it is better to cut or tear. Increase silica gel if the leaves are succulent. It is always advisable to use indicator gel if possible.
Silica beads work well too and are easier to reuse.
Never reuse gel without first making sure that no contamination from previous specimens is present. Both indicator and non-indiocator gel are widely available from florist supply houses in in most major cites in the world. Shake bag to distribute the gel between the layers of leaves. Store and ship samples in their bags or tubes in plastic boxes with tightly sealing lids. Refrigerate if possible.