A single speed bike is the easiest to wheelie but if you dont have one
lying around using the lower gears is what I find gets distance.
Using first and second are easy for yanking it up but you run out of gear too
fast and the bike is a little harder to balance at the slower speed with your legs pumping real hard.
If you can hoik it way up in 3rd or 4th to its balance point but peddle real
slow, with even a little brake if you have to (not the front one =:-P), that is
how you get distance.
Also, you naturally tend to torque to one side when you first start out.....
Try to concentrate on keeping the bike & bars perfectly straight when you
yank the front wheel up to the balance point.
I tried shifting a few times in mid air but busted my twig 'n berries enuff
to be very afraid of this technique =:-O
A good place to start and get used to the front end coming up if you are a complete beginner is to go real slow and yank the front end all the way up and jump off the
back of the bike while still holding the bars. You will learn how much it takes to pull it up but also how to jump off if it starts to go over backwards which it will do at some point in you wheelie career !
Once you feel out your bikes balance point (PRACTICE!)and are comfortable
all the fun stuff comes naturally......cross-ups, being able to make turns, flipping off your friends, etc...........