There are numerous variations in style; however a herbarium label should be on 100% rag paper and should contain the following information:
It is important to remember that the information gathered along with the specimen is just as important as the specimen itself. A collection notebook should be kept that details the collections' locality, habitat, water conditions etc. If necessary, it is possible to refer back to information in this notebook. When possible, the U.S. National Herbarium maintains collectors' notebooks along with the herbarium specimens for its major collections.
See the chapter by Tsuda and Abbott (1985) in the reference section for additional information.