Some tricks that will help. As stated above everything needs to be in good shape, battery, valve adjustment etc...
Cycle your glow plugs at least 2 times before turning the starter. After that Hold the accelerator petal on the floor while cranking, if it doesn't start after a couple of seconds cranking stop and count to 10, cycle the glow plugs and try it again at idle or just a little bit of throttle. Cranking at full throttle floods the cylinders with fuel and seals up the rings a little and helps on the second try, counting to 10 lets the fuel condense on the cylinder walls helping the compression especial on a old diesel.

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1983 300SD