Raman scattering is a relatively weak process. The number of photons Raman scattered is quite small.
However, there are several process which can be used to enhance the sensitivity of a Raman measurement.
Resonance Raman - can be used when the laser wavelength utilized is close to the absorption wavelength of the molecule. By irradiating the sample with a wavelength close to this wavelength an order of magnitude greater detectivity may be achieved. Not all samples will show resonance enhancement with common Raman lasers, but generally species such as porphyrins and those with a heavy central atom can show such an enhancement.
SERS Raman - SERS or SERRS Raman (surface enhanced (resonance) Raman spectroscopy) is arguably a less well understood enhancement technique. It requires a further moiety to be present (eg. A SERS prepared surface or colloid). The presence of such an agent can provide quite dramatic enhancements and has been used successfully in the study of biological samples such as DNA, peptides and proteins.
Active Substrates - here a specialized coating can be used to enhance the sampling sensitivity of a liquid or solution sample. The sample does not 'wet out' the surface remaining in a concentrated micro-droplet. Enhancements for such applications as assay screening have been shown to be a likely candidate for such technology.