Speaking as a daily driver, not a certified expert:
First - Replace your air filter. Not sure if it makes a difference, but I usually get the Mercedes air filter. Make sure the three rubber cushions supporting the air filter housing aren't broken. (Cheap to replace.)
Second - Invest in a case of Red Line fuel additive. Each bottle treats 200 gallons, so that's 10 fillups. The stuff's not cheap, but I take it as cost of enjoying large MB diesel sedans. For the first treatment, I would dump a whole bottle and take the car on an extended highway drive. Although an obvious point, do pour the additive first, then fill up the tank.
(I think there's also a reduction in diesel clatter noise.)
I buy this stuff also from the local MB dealer.
81 300SD, 193,000 miles.