What are some of the measures engineers would take to ensure that the LOX and other similar fuels are kept below their boiling point during flight?
As small amounts of LOX boil off heat is removed from the remaining volume. Boiloff actively cools the fluid and helps keep the remainder liquid. Rockets typically have vents to aid in managing the boiloff rate and fluid feed lines to replace lost fluid while waiting on the pad.
During flight the consumption rate outpaces any boiloff losses and it is not an issue.
Cryogenic propellants aren't used for long term functions in flight, so it is not an issue after launch either. Storable propellants like solid fuel or hydrazine are used when long term storage is necessary.
Superchilled rockets like the Falcon 9 cannot manage propellant temperatures as easily. As the propellant warms it expands before boiling off, so they cannot hold indefinitely while topping off fluids like a near-boiling-point rocket. Instead they must time propellant loading to end shortly before launch, and a delayed launch would typically result in a scrub while the warmed propellant is drained and replaced with cold propellant.
Adding to Saibogu's answer, ULA is currently developing technology called Integrated Vehicle Fluids (IVF) that will use boiloff gas normally vented during second stage coast phases. The waste gas will be used generate electricity using an internal combustion engine and also to power reaction control thrusters and pressurize tanks.