There are many ways to do this. How it's done depends on the technology your process can handle (can you place BGA components on a PCB, or is this project to use leaded components?). What efficiency do you need? A series-pass regulator will give 6% while a switcher can yield over 75%. Do you want to build just a few or is this for high volume production?
If you can handle BGA technology the Linear Technology LTM8050 uModule is a candidate. It is a non-isolated step-down regulator that needs just two resistors and two capacitors. It will convert 50 volts to 3.3 volts at 1 amp with an efficiency of 75%. The power dissipated is 0.825 watts.
http://cds.linear.com/docs/en/datasheet/8050fa.pdfIf you want a simple leaded component series-pass regulator a LM317HV with a PNP power transistor is a solution. The thermal resistance of the TO-220 package is 5 deg C/W and the dissipation should be limited to less than 10 watts with a substantial heatsink.
http://www.ti.com/lit/ds/symlink/lm117hv.pdfA standard LM317 can be used by placing a power-zener between it and the 50 volt supply. A 44 volt power-zener can be build using a power transistor and a 43 volt zener diode.
