Ideally start with uninfected plants through:
Buying from reputable dealer
Visual appearance i.e. no obvious symptoms of viral infection
Testing for presence of virus
Playing the percentages-
1) Vegetatively propagated plants are more likely to be infected than those produced from seed.
2) The longer a perennial plant has been in a green house the more likely it has picked up a virus.
Place newly acquired plants in an “isolation ward” to make sure no hidden pests or” not yet expressed” disease show up.
Space plants so they do not touch nor allow water drip through one pot into another.
Sometimes it may be useful to group plants so they may be treated as a single plant.
Sanitize all pruning tools before starting on another plant; this goes for harvesting flowers as well as trimming roots, too.
Wipe down your workspace with a sanitizing solution as well. This is especially important when repotting or dividing plants. Some growers when repotting only a few plants use a thick layer of newspaper for each plant (thick enough to prevent any sap/moisture from seeping through to the workspace) then gathering up all the plant and potting debris in it and start the next plant with a fresh pad of newspaper.
Wear latex gloves, and dip in a sanitizing solution between working on a new plant or new lot (group) of plants.
Never reuse old potting media.
If you are reusing plastic pots, they should be washed to remove all plant and potting material debris, and then soaked for an hour in a sanitizing solution.
This goes for metal or plastic stakes as well. Rinse before using or storing.
Unglazed ceramic pots could be sanitized by heating in an oven at 400-degree F. for two hours.
Sanitize your benches or tables on a regular basis.
Some viruses are vectored by insects or other animal pests. If these viruses are of concern, you may need to put screens on green house windows or grow in a screen house. Consider whether surrounding weed hosts may harbor an insect vectored virus of concern.