Inspect and clean everything that might go ashore before you leave home. Get soil, seeds, plant material, pet hair and debris out of boot treads, pockets, cuffs, seams, hoods, fleece, Velcro, backpacks, camera bags, trekking poles and tripod feet. Vacuum pockets and bags, and wash or brush outer clothing. Velcro is the worst offender for trapping seeds and needs the most attention.
Pay particular attention to anything recently used for hiking, farming, camping, birding or travel in another wild place. That's where the risk actually sits.
This isn't a one-time packing task. Your expedition team inspects and decontaminates gear before landings and runs a boot wash station, and you follow those procedures before and after every landing. Itineraries moving between South Georgia, the Falklands and Antarctica are the strictest, because the point is preventing movement of species between sites as well as into the region.
Practical consequence for what you buy: gear with heavy Velcro, deep treads and mesh pockets takes longer to clear.