I assume you want to prevent power flow back through the switching or linear regulator from a secondary power source on the output side of the regulator (EG:backup battery)
If the voltage drop on the DC-DC converter or linear regulator is greater than about 2.5 volts then the following circuit using an N-channel mosfet will work.
In the diagram the gate is connected to the power input side, drain to the circuit to be powered and the source to the regulator output. Here you can see the mosfet is not allowing power to flow back to the regulator.
The zener diode and gate series resistor are not necessary and may be omitted as long as the input voltage to the circuit is less than the mosfet gate breakdown voltage (typically 20V). The zener voltage should be greater than ~5V to allow the mosfet to fully turn on though it must be less than the mosfet maximum Vgs value. The resistor value is not critical.
The mosfet only needs a Vds rating greater than your output voltage so most logic level mosfets will work fine. Many logic level mosfets even integrate a zener diode for gate protection.
The difference between the input and output voltages is the mosfet gate voltage so this determines whether the mosfet turns on. If there is very little voltage difference between the input and output voltages, the mosfet acts as a diode and gives the usual 0.7V drop.
If you have less than 2V of voltage drop between the regulator input and output the following circuit should work down to around 0.5V of difference.
Resistor values are not critical. Reducing the upper resistor value to 10K or 1k with the same 100k gate resistor would improve operation for low voltage differences.
Both the above circuits should have very low reverse leakage current and be suitable for circuits with backup battery supplies. They are possible because the regulator input is accessible and significantly higher then the regulator output voltage. This makes it easy to detect whether there is a power source connected to the input using either a mosfet gate or a BJT as a crude sort of comparator. To switch between two power supplies whose voltages are very close an ideal diode circuit is necessary.