What's the best way of numbering invoices?

I'm self-employed and have never really known what's the best way of numbering invoices. I've come up with my own system whereby I number them with the date, preceeded by the client's initials, for example MB211207. It seems to work as I can always find what invoices I issued in any given month, but I wondered if there is a more efficient or beneficial way of doing it. Any business people have any advice?What's the best way of numbering invoices?
It doesn't matter if it works for you. We don't send many and just have a 5 number number stamp and put them on a list then file by invoice number and type the list into a spreadsheet and print a list by customer the sort and print by date or invoice number but we only send about 10 invoices a month at the most and each is backed by a file folder or box of back up data. When we send a partial invoice then we add a letter to the number for each additional invoice on the same job.What's the best way of numbering invoices?
QuickBooks is good. Very useful, but it costs about $200. However, everything is on the computer so it's easy to search invoices and manage accounts.

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