Here is the best way to do it bar none: Back up to the implement making no mistake on distance or evenness. Get off the tractor and give the line-up a once over. Once you are certain everything is perfect straddle the PTO shaft and pull into place.
Although everything was perfectly even and lined up the shaft will be off-center. Mutter a small obsenity about the impossibility of these relational physics. Get back on the tractor and go back and forth at least 3 times. Climb back down and straddle the PTO shaft again only to find out it is worse than the first time.
Even though you are fully aware of the weight of the implement you should now attempt to move the implement. This will automatically cause a slightly larger obsenity to blurt out of your mouth. Climb on the tractor again and while in 3rd gear rip around and get a clean shot at the implement. Climb off muttering something about putting a man on the moon and pull the PTO Shaft until it meets up with the tractor's PTO. Calculate the odds of six splines over the circumference of the shaft and after quick calculation make mental note to never play the lottery again.
Throw down the PTO shaft and kick the tire hard enough that you will remember it for the rest of the day. Wipe your forehead with your sleeve, yell at the dog, promise a kick in the pills to the unfortunate inventor of this contraption then go get a drink of water. When you come back the shaft will go on on the first or second try.
It is extremely important to put the PTO cover somewhere you will not remember it and it is likely to hide for at least 4 weeks. Climb on the tractor and wait for 20-25 seconds before you start the tractor. This will provide sufficient time for the wife to call you in or the cell phone to ring and avoid wearing out your starter prematurely.
I hope this helps...it works everytime for me.