1. A good dry top/trouser combo or dry suit. Keeping dry inside makes keeping warm that much easier. Even some of the two piece systems (dry top and dry trousers) available are getting close to being as good as a dry suit. Price is some much better as is the flexibility of the system.
2. Solid footwear. Wet river banks and slippery rocks are not a great combination if you have dodgy footwear. A good pair of river shoes or boots are worth there weight in gold. Currently using Five Ten Water Tennies or the new Astral Rassler.
3. Good socks. Conventional socks only really work if you have a dry suit with built in dry socks. However various manufacturers make thin 3 and 4mm thick neoprene socks that can help keep your feet warm even when wet.
4. Base layers. I love merino kit – it’s warm when wet and doesn’t smell! But any good, thin base layer will do the job. Colder = more layers. Polartec fleece also works really wet in cold/wet conditions.