There's some really good advice here guys, the most important being, if you haven't had proper instruction then don't try them.
The Barrell roll being half loop/half roll is a good analogy. You need to get the nose up very high (higher than the inexperienced imagine) before committing to the roll.
The most common error as has been mentioned, and is why it has tragically taken so many lives is rolling with the nose too low and spiralling in. It happens quickly. If you do find yourself in a nose low situation roll to the the nearest horizon (maybe by simply speeding up your current roll rate) and when wings level push/pull to the horizon.
As an aside, no manouvre involving pitching can be done at 1g (by definition!). Even a simple climb in a C152 involves pitching up slightly and thus aplying some g-force. Barrell rolls if flown well can minimise the g (in the z-axis) and keep it within the g-limits of non-aerobatic aircraft (but not within the guidence of the POH or insurance!).
If anyone would like specific advice on improving their barell rolls (or aileron rolls for that matter) please PM me. For ab-initio guidence though find yourself a good (and aerobatic experienced) instructor.
Regards,
SR