Techniques for Filtering a Noisy Buck Converter
Input filtering is a must. In addition to that I find it a must to use at least a full GND plane when dealing with switching regulators. Along with a full GND plane you can have much better routing for the other power connections.

Some additional things to go along with that:


Use every effort to keep all of the VSW connections as clustered together and as small as possible. Best is to have no vias on VSW.
Use a cutout under the VSW node and associated pads in the GND plane and any other power planes. This helps eliminate coupling into the power and GND planes.
Do not route any other signals through the VSW area on any layer. This node handles the highest ripple current at high frequency and can couple to other circuits very easily.
Try to design your board layout so that the switcher layout is not compromised in any way. Trying to place mounting holes and non-related connectors nearby is a compromise.