How to ensure DC input stays positive regardless of a positive or negative voltage input?
Yes. The bridge rectifier is the cheapest and simplest one.  

Only two diodes will be conducting for the whole duration.


A few things to consider:


If the required DC voltage is very close to the input DC voltage, schottky diodes can be considered because if their lower forward voltage drop.  Schottky diodes are bulkier and costlier than normal diodes.
If reverse leakage current is a concern. Normal diode can be used only at the cost of a little higher voltage drop.   
The voltage drop will be two times the forward voltage drop of single diode.  
If no voltage drop can be tolerated PMOSFET can also be used which drops only a few 10s of mV when properly chosen. The drop will be the load current times the on resistance of FET times 2.