13 years in business, and I always just sent out bills at the end of the month after the services are completed. I probably got stiffed for $1200 total ($660 from one customer that moved to Belize and wanted me to take care of their place until it sold - I would now demand payment up front for that sort of thing..) Most of this loss was in the first few years before I toughened up a bit.
To help with cash flow early on, I would prepare the bills a week before the month ended and as soon as I mowed the last time for the month I would send out those bills. That way money was already coming in before the first of the month.
I'm stunned anyone tries to collect each week or bills twice a month. I can't imagine any normal customer liking that set up. Customers that do like that are probably the chatty ones that are lonely and want to BS with you each week for as long as you'll let them. Don't fall into that trap unless she is stunningly hot and your pretty sure you're both single.
If someone is routinely late, it is time to get rid of that hassle. Eventually something is going to happen and they are going to fall further behind. Send out a nice letter with some facts. Say, "we count on regular payments to meet our monthly expenses. Four out of the last six months we have had to wait well beyond the due date before receiving your payment which has negatively affected our monthly cash flow and business goals. We would love to continue servicing your lawn but we will now require you to prepay for our work before any services can be rendered." Enclose a prepayment invoice for the following month plus any amount they currently owe you.
If they haven't paid, skip them. They will either immediately pay to get back on your schedule or they won't. Either way you'll be better off.
Lastly NEVER make any sort of threat you aren't prepared to follow through on. Threatening to take someone to small claims court over $100 dollars makes no sense. Is your time worth anything?? If they owe you money and they won't pay after reasonable nice collection methods, swing by each week and post an envelope with the over-due bill in it with a big red "PAST DUE" stamp on the outside. Nobody wants their neighbors seeing this, and it is even more effective when you have other accounts on the street. They will be worried you are telling their neighbors that they are deadbeats.
9 times out of 10 they will make you a priority. Very few of their creditors are at their front door each week. That gives you a major "squeaky wheel" advantage.
TO the guy that said he has 10K in outstanding invoices that go unpaid until winter rolls around each year: You sir, are completely crazy. The farther someone is behind the less likely you are to ever see your money. People that are always late on their bills are more likely to be forced to move too. When that happens it becomes way more difficult to collect. I'm sure you are a very nice guy but you are letting your customers walk all over you. Expecting (demanding) some moderate respect will make you come off as far more professional too. I can't imagine any possible advantage to running a legitimate business like that...