Like Erik says, decouple your power and signal lines where possible and practical. For your DS18B20 data line, I would not recommend to add any "large" capacitors as that will very destroy your neat square pulse shapes. What you SHOULD do is to put some sort of buffer in the data line, either 74/54-series 'Schmitt trigger' types or by just using two transistors. As a general rule one should prevent to directly wire CPU I/O pins into the field!