If you lightly drag the brake when learning it slows the lift and the propoensity to flip… Then when you do go to far its a very small movement on the lever which teaches you a nice level of control. Same principal for stoppies too
Always cover your brakes when wheelieng as you need the control once you get up some speed (after a lot of practice) – but yes a lot can be said for propoer body positioning and how you initiate the wheelie. Dont pull up with your arms as this can cause you to over-rotate over the back easily and cause you to veer off course. Use the pedals to initiate the acceleration and push the bars away from you so the bike pivots around your centre of gravity. And stay seated throughout
Wheeliing a fixed is the same principals as you agiain need to modulate your braking (slowing the pedals) in a similar way