If you come to the Slowinski National Park by car or motorbike, make sure you have enough petrol. The nearest petrol stations are at Ustka or Slupsk, about 40 km away from e.g. Kluki.
There isn't much traffic in the area, not even in the high season, so you cannot count on other drivers to lend you some petrol and may find yourselves stuck in the middle of the beautiful countryside without any chance of going on. This may be quite romantic, but for how long?
Update: I've just heard from robertsz (thank you, Robert), who says that there is now a petrol station at Smoldzino opposite Rowokol. He doesn't say though what its opening hours are and if it's open at weekends.
You needn't fear them on the beach, mosquitoes can't compete with the wind there, but the closer you come to the lakes and marshland the more of these biting intruders will follow you, trying to feed on your blood. The places most notorious for a whole plague of them are: the forest on your way from Leba to the shifting dunes, the marshland around the lakes and Mount Rowokol, especially in the evening. So remember, mosquito-repellent is an absolute must in the Slowinski National Park!