I ran across this while I was researching something else...naturally.http://www.answers.com/topic/barrel-roll-2
It gives a pretty good coverage of barrel rolls and all their variations including some nice animations.
Two things that I think are worthy of pointing out,
1. Unlike what we said, some airplanes will require rudder to complete the roll. This is logical enough as many, particularly early prop, planes don't roll all that well by aileron alone. Once you move into jets however, rudder use becomes more the exception than the rule.
2. One thing the article suggests that I think is a bit misleading, is that you should commence rolling the aircraft shortly after your nose breaks the horizon.
One of the big lessons to be learned from the Barrel Roll is lift vector placement.
In other words, where goes the lift vector, so go you.
Your goal in the first portion of the BR is to get your nose 45 degrees up, against the force of gravity. You also need to get your nose 45 degrees off heading, but you will see that that isn't very difficult. If however, you begin rolling immediately in the BR, you will rapidly divert your lift vector from the vertical and you will find it increasingly difficult to get your nose up where you want it. Remember, after only 30 degrees of roll you have already lost about 13% of your lift; the rest being diverted into the horizontal. By 60 degrees, half.
So, in order to perform a good BR, I'd recommend rolling not at all until you get a good 30-35 degrees of nose up. Then you can play out the remainder of of your angle of bank to reach 45 degrees up when you reach 45 degrees off heading.
This is not a pointless lesson restricted to airshows pilots. If you want to get your nose up to do a turn reversal, do it wings level. If you are at the bottom of a rolling vertical scissors and you are trying to minimize the vertical portion of the overshoot, do it wings level. If your nose is buried after a bombing run and you are trying to get back up above the threat envelope....wait for it....yes, do it wings level.
In cases other than the pure vertical, the lesson still holds. If you want to get behind someone, don't put your lift vector (or velocity vector if you aren't turning) in front of them. This is why continuing a high aspect attack by continually leading a bandit yields a (generally ugly) overshoot.
Hope that helps.