Great rules for two-up posted by all. As far as getting started... review the guides posted here and practice the same way you learned to ride. Start in a parking lot where you can do sweeping turns in both directions. Practice starting and stopping and see how your distances are affected by the additional weight. Then practice taking right and left turns from both moving and stopped positions. Try and practice as many real world situations as possible in the parking lot or on deserted streets. Then gradually move into the population and take short trips around town. Biggest things I would add to the guides above:

1. Stopping distance is going to be affected - plan to start stopping sooner
2. More throttle to get started - especially on a hill
3. Passenger may slide forward or back depending starting or stopping, both should be prepared for some minor shifting.
4. Slow speed anything will be affected
5. When stopped, minimize the shifting of the passenger unless you are ready for it.

 

My 2 cents --

1. Get very comfortable with your bike first. I have been riding since the 1970's, but wouldn't 2-up until I got a few hundred miles on the new bike.
2. Like several have said before, get an experienced passenger if you can. A passenger that sits still is prefereable. Even a little figet results in little s-turns. A passenger that leans over to get a better view can cause you to drop, espescially at low speed. (don't ask how I know ) Passengers shouldn't be thinking about leaning. Best way I've heard is to tell the passenger to close their eyes in a turn. The exact amount of "lean" feels like normal sitting when you can't see the horizon tilting relative to you and your bike.
3. Expect helmets to bump occaisionally, espescially when braking or downshifting. If you do not have a high-rise passenger seat, this happens a lot. Let the passenger know this too. You don't want them to react instinctively to an otherwise harmless bump of the helmets.
4. The bike is heavier, and the weight is shifted more to the back. Adjust the shock preload, and increase rear tire presure per the owner's manual to compensate for the extra load. The bike can't corner like it usually can, so the back wheel will lose traction and step out if you take a turn at normal speed. Take it slow around turns until you get used to it.
5. Get on the bike first, instruct the passenger to get on when you have the bike balanced and ready for them. Instruct them to approach from the side AWAY from the pipes. Have them put a hand on your shoulder to steady themselves as they step up onto the passenger peg while you hold the bike steady. Hold both grips and apply the front brake the whole time while they climb on.

 

when i start my buddies out taking peopel on the back for the first time... i usually suit up and ride bitch with them first around the parking lot so that they know what is gong on... i look at it as i would rather go down fully geared up and knowing how to take it than their girl... as gay as it sounds and looks, it really helps the rider to become comfortable wiht a passnager... so wehn my 200lbs *** gets off and their 100lb girl gets on it is like they are not even there!

 

I learned this back in the day of riding pedal bikes.....Having a person sitting on the seat & another on the handle bars....Then progressed through all the various sizes of dirt bikes I had & friends wanting to tag along. So I guess it kind of came somewhat natural to me when it came time on a street bike. But I'd agree with the others and JRK below, get an experienced rider to practice with before attemping it with a newbee passenger. Get to know your bike solo really good before attempting to 2-up.