Bench vise....

Anyone doing much bike work at all and that has even a modestly set up permanent shop really owes it to themselves to get a decent bench vise. The vise is your best friend with the third hand you'll ever have.

If you don't have a permanent bench setup and have to work out of a tool kit which has to be put away to clear the room to serve dinner then that's different and you're stuck with having to resort to oddball tricks such as holding a wrench or vise grips between your knees. But with foldable benches such as B&D Workmates and the like on the market there are better options than contortions to do this.