I have a closed circuit orange SMB for normal use, and keep a 65kg (about 150lb) liftbag on me as well (I know its a bit big but I won it at the dive shop christmas party). Both are clipped off to the rear d-ring on my crotch strap, just below the cylinders, a la 'DIR'.
I usually keep a spool attached to the liftbag all the time, and just attach a spool from my pocket to the smb. The reason I do this is if I need to use the liftbag, it means I have lost the SMB, therefore I have a problem, and may be starting to get stressed, so I do not want to be fumbling, in mid-water, with gloves on, trying to attach things. I just want to unfurl, inflate, and calm down.
As has been stated before, here in the UK, most deco is done on the drift, with the boat following. No my usual skipper is one of the best, and sometimes he has problems due to weather etc. with seeing liftbags. In fact, if it is really bad he has probs with SMB's (8ft waves a couple of weekends ago, just blew up while we were on the wreck - made the 6m stop a bit lively!!). In that situation, out comes the flag. It is lighter, longer than the SMB (6ft) and bigger (the material is 3ft by 2ft). Because it is lighter it can be waved about easier.
Just my way of doing things to suit my conditions and diving style