Seeing as viral titers are expressed as virus per volume, you can concentrate the virus in by reducing the volume. We usually do this by ultracentrifugation the virus through a sucrose cushion and then resuspend it in the volume required for the new viral titer concentration.
In addition to the above suggestions you may passage the virus in a relevant amplification host cell line (eg. Mosquito epithelial cell line for arbovirus) once or twice. This may help in enhancement of virus titres. Try to collect the supernatant before the onset of cytopathy.