Whether you be a teenager trying to make some cash this summer, a husband trying to best the Jones' home, or just plain anal retentive you know or will discover that lawnmowing requires more than just a bottle of water and a tank of gasoline, but a little bit of science and even real skill.
Things You Will Need
(1) Full Tank of Gas/Battery: As mentioned; many good intentioned husbands on Saturday afternoons have reverted back to their sedentary position when the daunting task of going to the gas station down the street is required making an additional thing you don't want to do with your 'me time' so come prepared and always make sure that you have plenty of gas kept in a proper storage container. Refilling your container regularly, when at a quarter full or even half means you'll never be empty. When it gets low just throw it in your trunk and don't forget next time you fill up your car.
A more 'Green Solution' for your green lawn would be getting a battery powered mower. If you purchase one of these pay attention to what the battery life is. Most don't have enough juice in a single battery to finish most suburban lawns so you'll usually need backups. Make sure you recharge them after each mowing and/or in accordance with the manufacturer's directions.
(1) Bottle of Water: As mentioned; you need to keep hydrated and doing so will make some of the larger lawns much less exhausting and as a result the time will fly by.
(1) Pair of crappy shoes: Whether they are an ugly pair someone got you or just some cheap ones you found at Walmart, it's best to have a throwaway pair instead of scrubbing the green stains off every week.
(1) Lawnmower: Well, obviously. Whether push or riding or battery powered, definitely would recommend a lawnmower for the task. I've seen obsessive men and punished children both clip the lawn with scissors but if you're looking to finish the lawn and still have the weekend left to you, I would encourage going the motorized route though some especially small yards can be trimmed with just a weed(wack/eat)er(whatever you want to call it).
Also: Sunscreen, hat, bugspray, allergy medication, as needed to remain comfortable and not be a burnt, itching, sneezing mess. When all the above is used correctly, lawn mowing can be a relaxing time even to meditate or listen to some podcasts you're backed up on.
If at first you don't succeed: Pull, pull again
Most mowers need you to manually pump (usually find it near the fuel tank) and then rip that gosh-awful cord. If doing so for the first time, you will almost definitely fail. Some mowers are trickier than others to start and require a variety of handles and levers to be in the correct position while you use your other remaining limbs (you are a spider human mutant, correct?) and concentration and strength to plant yourself firmly on the ground and pull...and pull again...and again...and rest...and again and then finally, the miracle moment and you hear: "Gu, gug, gug, gug, gugr, gugggerrrrrr."
A New Direction
Your lawn may be square or circular or rectangular or split up into three dimensional polygons and hills and you may have mow sideways to avoid those delicate plants the wife has growing (wait, those aren't weeds? They sure look like weeds) and mow around the pool and avoid the landmines of dog droppings and stay far away from that property line or else Jones will be so offended at you mowing a blade of his grass that you know that side of your house will be covered in egg come morning; the point being, there is no set most efficient way of mowing the lawn because there are so many different shapes of lawns and even different kinds of grass. Additionally, if you're told that mowi vertical lines over horizontal or something of the sort makes the grass grow better or quicker or whatever, know it's just an old wives' tale. There is no logical reason why this will effect grass growth but if you are mowing someone else's lawn don't argue it.
Experiment a little, try mowing the outline and gradually working in. Mow diagonally, mow horizontally. If you are mowing someone else's lawn find out their preference. Some require their lawn to have a certain look and if they walk out and see squiggly lines criss-crossing all over their frontyard they might not have you mow again next week. Ultimately, see what works and try to make it varied enough so that you don't get bored.
Give yourself a break
Find accomplishment in what you have done. Look down the street or just next door and compare the lawns and bask in your glory or possibly well earned cash. Before you plop yourself back down on the couch though you may want to change and shower or you just might get bits of grass all over as well as the smell of it along with your own natural odors that have been brewing for the past hour baked in with a layer of gasoline.