Yeah I usually just send a statement after an initial invoice. I go back to the previous invoice or when they last had a zero balance. Most of the time when I send a statement i'm sending out a new invoice along with it. I staple them together with the statement on top. If they got all confused or something I will go back further with the statement. That happens sometimes when the bill is the same amount as another or if things cross in the mail and my statement shows something they already paid.
I've gotten a few calls in the last couple of weeks because I sent out invoices for all work done in October on Novemeber 1st and then if I did a final cleanup in November I invoiced that without waiting until the end of the month. If they had an outstanding balance from the November 1 invoicing they got a statement reflecting that invoice as well. Personally I don't see what's confusing about that. They always get a detailed invoice for any new work....date of service(s) and services provided.
P.S. - Generally I don't send a statement going back very far unless they request it. I don't really want them seeing just how much they've paid me, lol....OMG...$1,500 this year just to have the law mowed!