I didn't really read everyones comments on here, so forgive me if I repeat anything.
1) Offer a 5% discount for customers who pay for a year in advance. Great way to start the season off with some money behind you.
2) Offer packages. Start w/ just offering weekly cutting, spring/fall clean up, and fertilizer year round for your basic package and then offer more services as you go up. Next, in addition to your basic package, you could offer weed control of the lawn/beds plus hedge trimming twice a year. The great thing about offering packages is that you can lump everything into one big price and divide it out over the course of a season, thus increasing what you bill out monthly. So you're essentially getting paid for future work. Plus it's a great way to sell extra services. Just make sure you price it right from the beginning!
3) Start paying your employees at the end of the second week of every month. For example, employee A works 40 hours a week at $10 an hour. By the end of the second week you pay him $800. Third week you pay him $400, fourth week you pay him $400. Repeat cycle. Now you're probably asking yourself "Why the hell would I do this?" Well, I'll tell ya why. In my experience, the fastest a customer can really pay you by mail is a week. About 3 days out and 3 days in, right? So lets say you only receive about 10% of your money by the end of week one. That's another week you need to cover in labor expenses, in addition to the month you've already been paying out and no money coming in. Now you have to pay a fifth week of labor and you're still waiting on most of your money. That second week is HUGE. I would say I receive about 60% of my money by the end of the second week. Now you're in much better shape to cover your labor costs. And I'm sure most people would tell you that labor is probably there biggest expense. Oh and also in case your wondering why not just pay every other week. Well some months have 5 weeks and that just throws everything off.
4) Auto Pay.. Bill goes out.. money comes back in a fraction of a second. Can't beat that.
5) E-mail/Online billing. People are MUCH more likely to pay faster when receiving an e-mail or simply going on your website. It's just more convenient in todays society. Plus think of all the people who receive bills and just throw them on the desk/table getting lost in the clutter.
6) Diesel Mowers. Save on fuel costs like you wouldn't believe.
7) Electric Weed Whackers. No gas what so ever.
If you start offering the pay in full, the packages, auto pay and email/online billing you'll probably increase your cash flow by 50%. Theres a good start.