A lot of David Grist's stuff is big and bold so you can learn multiple balloon sculpture without needing to learn especially advanced techniques.
The balloonhq columns have a large number of sculptures that you can learn from. Also browsing the photo section will inspire you to try more advanced models.
Attending a balloon convention is another great way to get up to the next level.
I personally learnt multiple balloon sculptures in isolation. (I thought I was unto something new until I got on-line).
Using the techniques you've learnt for basic sculptures why not just see what you can come up with using several balloons.
George
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