I still have one of my original buckets from when I started brewing 15 years and almost 300 batches ago.
My SOP is to soak in alkaline cleaner, using a kitchen sponge (not a scrubby) on any tough spots. On brew day I sanitize by mixing a gallon of iodophor, sealing the lid, and splashing it around for a couple minutes. Once a year I do a sanitation check by making an extra-large batch of starter wort, pouring about half a gallon into each bucket, splashing/rolling it around to contact the entire surface, and letting it sit at room temperature for a few days. If there's no gravity drop or visible bubbling/pellicle/etc., the bucket can still be effectively sanitized.