  We report long-term scintillation observations of 18 pulsars in the DM range
3-35 pc cm^{-3} carried out during 1993-1995 using the Ooty Radio Telescope at
327 MHz. These observations were made with the aim of studying refractive
effects in pulsar scintillation, and obtaining reliable estimates of
diffractive and refractive scintillation properties. Dynamic scintillation
spectra of pulsars were regularly monitored at 10-90 epochs spanning 100-1000
days. Significant changes are observed in the dynamic spectra over time scales
as short as a few days. Large-amplitude fluctuations are observed in quantities
such as decorrelation bandwidth, scintillation time scale, drift rate, and flux
density. Several pulsars show organized features such as drifting bands in a
highly pronounced manner. For some pulsars, gradual and systematic variations
are seen in the drift rate of patterns which undergo several sign reversals
during the observing time spans. Anomalous behaviour such as persistent drifts
lasting over many months are seen for PSRs B0834+06 & B1919+21. Four pulsars
were studied for 2-4 well separated observing sessions, each lasting over ~ 100
days. In some cases, significant variations are seen in the average
scintillation properties and/or flux densities between successive observing
sessions. From our data, we have been able to obtain more accurate and reliable
estimates of scintillation properties and flux densities than those from the
earlier observations, by averaging out the fluctuations due to refractive
scintillation effects. These measurements are used to derive parameters such as
the strength of scattering and scintillation speeds. The present measurements
are compared with earlier measurements and the long-term stability of
scintillation properties and flux densities is discussed.
