Manuals are the Holy Grail. There is a big hill I climb on my road rides around here. (On my Bullitt) I pedal up into my big ring and pull a manual all the way down the backside...probably around 35 mph,
for 1/3 of a mile! Cars will never pass...they follow in amazement!
just gotta soack it up a ton with your legs and then push them back and extend into your normal manual position again. If your talking about doing it on a DH bike, then good luck, its hard as hell (usually the rear shock will bottom and totally throw you off balance)
ya i find it easier to manual with taller bars and of course a shorter rear end helps so if u can move your rear wheel a little further forward that might help. if i'm having a good day i can usually manual for about 20 feet after bunnyhopping up a curb. the best thing to do is pull up pretty hard but not try to hop too much and just kick your legs forward to keep the wheel up.