One thing that works for me that I've not yet seen mentioned is physically splitting up the players. I found that when the party split but all players stayed at the table a lot of information that shouldn't have been available was used. Players used very poor justifications for their characters' sudden decisions to rejoin the party at times that were extremely convenient to the action. They could parallelise with near-perfect knowledge and near-zero risk, and they loved it.
So housekeeping stuff goes through on the nod, as Tim Wiseman suggests (thus speeding up boring paperwork, yay). Shopping (at least in civilised areas where law and order holds sway) is very quick: they can have anything they want from the PHB at 50% over listed price (the markup reflects not haggling, but risky orplus they look like they've got money). Risky, speculative, or role-playing stuff, however, requires physical separation of the subgroup. If it's one or two players, I'll pick up what I need to resolve the immediate issue, and ask the players to join me in another room. If it's more than a significant subsectioncouple of players I'll ask all the other players to leave until called for.
Firstly, this imposes real information separation. I ask the leavers how long they're planning on being away as they leave, and they won't have any opportunity to participate in anything but their own subgroup's thread until then, unless there's a valid in-game reason. I run the other subgroup's thread forward until they decide to rejoin / I need to know if the other group have done something in the interim / that in-game time is reached, then those in the room leave, those outside return, and we run their thread forward until a similar breakpoint is reached. Often, this leads to unforeseen plot developments, and consequent in-game tension ("The wizard told us to meet her at the soap stall at dinner time, and when we got there the stall was a smoking ruin surrounded by a four-foot high wall of bodies. What did she do?"). The players are genuinely wrong-footed, confused, scared, and consequently excited and fully-engaged with the plot.
Secondly, my players now expect this, and don't always like it, so they are incentivised not to split the party unless there's a really good reason to do so (of course, I can still decide to split them!).