Permethrin is the answer for me. I wear rubber boots with my pants tucked in, which also helps a lot. If you’re wearing lace-up boots, add boot gaiters.
A less effective, but still useful, tactic is to get in the swimming pool or hot tub immediately after potential exposure. It’s anecdotal, but we always used to get fewer ticks and chiggers if we’d jump in the pool for 15 minutes after picking blackberries than if we went straight in and showered.
Once you get them, use whatever anti-itch cream you like for poison ivy or other itchy things. Contrary to popular belief, chiggers bite you and then fall off before the skin even reacts; they aren’t burrowed into your skin, so old remedies like putting fingernail polish on them to suffocate them don’t work.