You can sample probably 6 or 7 times as much material if you put it through a screen before you pan it out. All the big junk is gone and you will be looking at the most likely material to contain gold.
There are tons of good classifiers for sale or you can use any of the free plans available and make your own.
It is fast and easy to use a screen and sort the gravel down to a size that is best to hand pan. The other nice thing is that you will be more likely to see some gold in your pan once you have classified and panned your samples.
You can do your screening at the site and put the resulting material into a labeled bag and carry it back to camp to do the panning. This will give yo an idea where to go the next time you head out.
Always label and indicate with a drawing or map where yo tok the dirt from, include water level if you are gathering from a creek or river. If you can you should try to mark the site to make it easy to find when you return. Don't put up a sign but figure a discrete way to mark so no one will notice and start wondering what is happening.
I am linking here to a couple of good classifier plans.
