A Short Introduction to Paleography from the Archives at the University of Southampton is a brief guide to help you understand some of the quirks of handwriting in old documents.
Paleography web tutorial from the British National Archives focuses on English documents written between 1500 and 1800.
Transcription Tips from the U.S. National Archives gives you tips and best practices for reading old documents.
Study Guide Colonial American Handwriting from Reed College is a fun interactive way to learn about early American handwriting practices.
How to Read 18th Century British-American Writing has best practices and tips for reading old handwriting.
Reading Early American Handwriting by Kip Sperry (Seymour Library Main Stacks Z115 .A5 S641998) has chapters on abbreviations, dates, and sample alphabets, plus over 150 pages of early handwriting with transcriptions!