I change the first three characters to read XX0 (Where XX are my initials and the third character is zero). Then, when the numbering goes "over the top" I change the third character to "1", and so on. That way I get a consistent numbering up to 100,000 images (less one!).
If you changed that third letter to a sequential letter instead of a sequential number you would have 26 characters for that 'place'. This yields 260,000 distinct names for images. I use the first two characters to identify my various cameras so I can tell at a glance which camera was used. I do not rename my pictures but store them in folders which have dates and tell me what the pictures are.
Good thought, Jim! I also store my files in folders whose names begin with the date. I write the date in 'backwards' format (eg 20120708 for 8 July 2012) so that Windows Explorer shows them in chrono order.