Anything you do to a particle will disturb its wavefunction in one way or another.
If you perform only a slight partial measurement (only learning a little about say, its position), you disturb the wavefunction a correspondingly little amount.
So, you can reduce the number of possible locations of a particle, but the more you narrow down its position, the stronger that measurement disturbs the wavefunction, and the less you'll be able to learn about other observables (like momentum).
Wavefunction "collapse" isn't necessarily an all-or-nothing phenomenon, but can occur in degrees. Measurement can be described as a physical interaction between object and measurement device, also covered by the laws of quantum physics.
As one neat example, it's possible to measure both the position and momentum statistics of a beam of light, by doing multiple partial measurements in position, while measuring strongly in momentum.
See for example:
http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.112.253602