It is hard to provide any suggestions without more information. What are the fields involved? How do you differentiate one work order from another, so you know when to increment?
I could use a python script in field calculator to create sequential numbering as shown in this example ( How To: Create sequential numbers in a field using Python in the Field Calculator ) but if a record is deleted then it renumbers the whole data set.
I could use the mentioned script as a starting point but how could I modify it so each new record is number based on the next number in line?