What Are Causes of Swollen Feet?
There are several possible reasons why a person's feet can swell after a hike. Exposed feet can be stung or poisoned by the different flora and fauna that inhabit the environment you're hiking in. Ruling that out, assuming you wear protective footwear, we're left with "Hiking inflammation" which is basically just a fancy way of referring to the way your foot swells up because of overuse.
The way swelling works is when a particular section of muscle is overworked (In a hikers' case, the feet), the blood vessels in that place widen up, allowing more blood to supply that section to keep up with the heightened demand. This increased volume of blood in that area causes the darker coloration and warmer temperature.
The blood vessel walls, being inherently porous, allow some of the blood cells to leak into the surrounding tissue. This leakage of excess fluid can jangle the nerve endings in the area, which causes pain signals to shoot up into your brain. This onset of pain isn't exactly a bad thing. It's just the body doing preventive maintenance, warning you to hold back a bit so as to not do any serious damage to your foot. Your body takes this downtime to do some damage assessment and control and toughens up the muscle tissue in that area to be more resilient in the future.
That being said, though you may be a veteran of many a march across all kinds of terrain, you aren't exempt from the onset of swelling. Seasoned hiker you may be, but your body still has a limit. I've been hiking for a little over 7 years, and I still find red-hot swelling feet under my heavy-soled boots and double layers of socks. To ignore the swelling and insisting on pushing on could lead to further, more painful complications, least of them chronic inflammation of the feet, which can rid you of your hiking hobby for a couple of months at least.
What To Do Once Swelling Sets in During or After a Hike
FOR STRESS-INDUCED SWELLING
The first thing you want to do once your feet start groaning in a complaint of carrying your weight all day is alleviating them of that burden. If you couldn't bear the pain enough to walk all the way back to your car, drive home and rest, sit down on the spot and elevate your feet so as to minimize the amount of weight it's supporting. Remove your shoes to let your feet breathe. Keep them rested until you're certain that you can walk on it again without too much trouble.
Ice packs or any frozen items can be used to make the swelling go down. If none are available, find some snow to apply to the affected area or a cool body of water to dip your feet in for 20 minutes to numb the pain. Some people find that heat is more preferable to use for that purpose, but personally, I prefer cold water.
SWELLING FROM OTHER FACTORS
Some foot swelling isn't induced by overuse. Sometimes they're caused by health problems, which should prohibit you from taking up hiking in the first place. Make sure that any swelling isn't a more serious medical issue before taking up hiking. Other times, however, swelling can be caused by insect bites or poisonous plants.
You can easily tell this type of swelling is not induced by stress mainly because of this kind of swelling itches. This can be easily prevented from affecting the feet and other parts of the body by wearing protective clothing. Some swelling is also induced by allergies. Packing some anti-inflammatory and antihistamine medications with you should help you get rid of the swelling brought on by these factors.
In order to hike without being obstructed by such petty trifles as hiking inflammation, it's best to do a little preventive maintenance. I find that taking those anti-inflammatory and antihistamine meds before the hike as well as when coming into contact with inflammation-inducing factors helps to increase the chances of avoiding the onset of swelling altogether.
In addition to that, doing some pre-hike stretches helps condition your body for the physical stress it's about to undergo. The next time you're about to go for a hike, try doing some simple leg exercises to stretch out your muscles. I used to do a little exercise in which I held on to my left foot, pressing it up to my buttocks as I bent my left knee inwards. Do that for both feet for up to 20 seconds. Couple this little exercise with some other stretching exercises, and remember to keep it simple to avoid actually straining your muscles before you even hike.
Other than that, always remember to hydrate frequently and take in a lot of good carbohydrates before and after your hike. Some people like using menthol patches on their feet and legs to help avoid swelling, but personally I like to use vapor rub before and after a hike.