Go with all numbers or all letters. If you must mix it up, then make sure there are no ambiguous characters (Il1m, O0, etc.).
When displayed/printed, put spaces in every 3-4 characters but make sure your systems can handle inputs without the spaces.
Edit:
Another thing to consider is having a built in way to distinguish orders, customers, etc. e.g. customers always start with 10, orders always start with 20, vendors always start with 30, etc.