You definitely want to create an invoicing system because you are going to need that for anything you do in life. It doesn't need to be complex. It just needs to work.
What I would do is make a master folder called CLIENTS for all my clients. Then create a sub folder for each one with their name. Simple, easy, you know where to find their things. So that is step 1 and very easy to think about.
Step 2. You want an invoice template. This is just a word document with your name and address at the top, and then the client's name and address a little further down on the left. Then just a statement on the page about "this invoice is for the amount of $xxxx for the following project:
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and list out what you did that month. If it's the same every month then that's even easier. So if you want help creating an invoice template, let us know. We can help with that! That should also be fairly straightforward.
So then to create a new actual invoice, open that template and save it out with the date of the invoice. So maybe inv2010-03.doc for example. That way any time you want to see what you've invoiced you just look in the directory and you see *exactly* what months you've created invoices for, for a given client. It makes all of that very easy to track.
If you're not sure if you should send someone an invoice, because you don't have a record of their check (a common issue), then just write them and ask them for the check number, for your records. They can then write back and tell you the check number, so you know they gave it to you. Or they write back and say "Oh I haven't given you a check yet" in which case you send them an invoice.
