All the time! I think the only way to be certain the seeds don't spread is to burn all outer garments. I've seen people pick the individual seeds off but it can take hours, most people can't be bothered and it's easy to miss a few. Accordingly, if you know there are invasive species with clinging seeds in an area you are working in, disposable outer clothing is the best option. Thorough cleaning of boot soles is also very important, especially where there are rhizomes of plants like Japanese knotweed that can easily get stuck and transferred.

VEHICLES: The issue is not when you get seeds ON YOU  while WITHIN the protected area, but people coming INTO protected areas with weed seeds on their VEHICLES, especially off-road vehicles.   Seeds on vehicles is what is spreading a new weed Saharan mustard across millions of acres of our desert iwthin only a decade., that you can see at http://www.ecoseeds.com/mustards.html

ROAD SIDES. How you manage the road sides at the edges of protected areas, and roads going through protected areas, and trails, railroads edges, and domesticated animal manures?  These alll help spread weeds into protected areas very much faster than any seeds carried in by humans on their clothing or shoes.   Even areas that have been protected 150 years ago like Yellowstone National Park or Yosemite National Park, weeds can travel along roadsides or by cattle in their manures, into those areas very easily. 

INTENTIONALLY SOWN SEEDS. Also intentionally sown exotic seeds, like along roadsides or in nearby agricultural areas, also can be a huge source of weeds.  If you look at my mile-by-mile survey of the western USA that I did in 1997, it was not weeds that were the big problem in most States, but intentionally sown exotic seeds along the roadsides or escapes from nearby exotic-sown pastures, at http://www.ecoseeds.com/megatransect.html. 

UNDERSTORY SPATIALLY EXTINCT OR DAMAGED, SO NOW VULNERABLE.  In 2015 most semi-arid to arid lands across the planet, the native plant understory is spatially extinct or severely damaged, to the degree that bare open spaces create vacancies for any new exotic plant to get established.  

That is how California became the most weedy spot on the planet 150 years ago, the cattle ate every bit of native under story during the 1864-65 drought, causing spatial extinction of the native under story, and the resulting bare land got colonized with close to 100% exotic cover within a few decades.

DO A VEGETATION TRANSECT.  If you have less than 100% local native cover (including native plant thatch) in your vegetation understory, then those areas will be eventually invaded by one or more weeds in the future, as open spaces have been created to let in the new invaders.  

If you do a simplified Evens & Love (1957) Toe-point transect, where you walk 100 paces and each time you step, you note what plant your toe is touching, or you note when you hit bare earth--what percentage is native plants, exotic plants and what percentage is bare earth?  

PERCENTAGE OF VULNERABILITY. The percentage EXOTIC plants s your CURRENT vulnerability percentage and the BARE EARTH is the FUTURE vulnerability percentage of the area, for exotic plant invasion.  

Hope this information is helpful, and hope your land does not end up like California, a 99.999% weed-covered under-story. 

Yes, this is common.  My staff and I work in protected areas.  We are constantly concerned about transporting invasive species from one area to another.  A good prevention program should include public education, prevention measures, early detection and response monitoring, and integrated pest management techniques.  We frequently work in wetlands and wear rubber knee boots, which are relatively easy to clean and have few places for seeds to cling.  We clean clothes and equipment before leaving an area and dispose of seeds and other potential propagules (e.g., sections of rhizome) in the trash or using a bleach or other biocide solution.  Below are some resources I think you will find useful:

Invasive Plant Prevention Guidelines

http://www.weedcenter.org/store/docs/CIPM_prevention.pdf

State of Michigan, Invasive Species Decontamination for Field Operations in Michigan

http://www.michigan.gov/documents/deq/qol-wrd-policy-invasive-species-decontamination_476846_7.pdf

Global Invasive Species Programme, Guidelines and Toolkits

http://www.issg.org/gisp_guidelines_toolkits.htm

I agree with all of the above.  What you take in (to an uninvaded area) and what you bring  out (of an invaded area) are both important.  And vehicles, tools, boats, etc are important, in addtion to clothes. Lots of great internet pictures on this topic; here are some:

wash everything!

http://www.nature.nps.gov/biology/invasivespecies/Prevention.cfm

wash boats!

http://missoulian.com/news/local/missoula-fly-shop-sets-up-boat-wash-to-fight-invasive/article_3e6a4dae-b9b9-11e1-9355-0019bb2963f4.html

http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=11340

wash vehicles!

http://nwipc.org/about-us/projects/atv-wash-station

http://inciweb.nwcg.gov/incident/photograph/3004/4/

http://www.wiley.com.au/news-events/new-wash-down-facility-ups-the-ante-on-weeds/

wash your boots!

http://fl.biology.usgs.gov/c1258_Dodd/html/biosecurity_and_disease.html

http://www.nps.gov/sagu/learn/nature/how-you-can-help.htm

don't wear felt boots!

http://juneauempire.com/stories/022111/reg_788567875.shtml#.VW-mfka06-Y

Yes, particularly if any body is walking inside protected areas, those thorny species of plants seed  left with cloths.

Yes, I find seeds on my clothes and shoes very often. Especially grass seeds disperse from one place to another by catching a ride in the vehicle's grid in front of the radiator. Except for when I wash the car, I never remove seeds from the vehicle, which creates ample opportunity for unintentional dispersal. Seeds attached to my clothes are removed by hand and either dumped in the field, in front of the research quarters, in the rubbish bin or inside the fire (when I feel particularly hostile towards that kind of seed... e.g. thorns). Policy recommendations: I do not have anything to add to previous comments. In short, limiting unintentional seed dispersal will be very hard. 

Of course, all the time! One cannot burn all outer clotjing after each lield trip (budgets won´t allow for that), so one can only try to pick all individual seeds off one's clothing, and never forget to detail one's vehicle. Seeds should be burned, or crushed.

Thanks for the wonderful answers so far..It seems obvious that same small hygienic practices can help limit unintentional seed dispersal. 

I found thousands of native grass seeds in the grid of my cruiser in the Kalahari and transporting these from protected  to unprotected areas and vice versa. I burned the grid for removal. Not to prevent spread, but to keep my cruiser on the (off) road.

Anyhow, as argued elsewhere in the Q&A, seed dispersal is often not the driver of an invasion, but the receiving environment.