I make sure the shoes I wear are properly broken into and appropriately padded (I sometimes buy 1/2 - 1 size larger, then fit additional insoles inside).

Take breaks during the day, if only for coffee or a snack. It gives your feet time to rest, plus in Europe, sitting around is the best form of sightseeing.

At the end of the day, I usually soak my tired feet in hot water for 20 minutes or so. I've found that it also helps A LOT to prop my feet up on the headboard (while laying flat on the bed, without any pillows) for about 30 minutes.