This is a difficult thing to do and seems to drive me nuts at times. It almost makes some of my prospecting trips unenjoyable. Either I'm constantly thinking while I'm on my prospecting trip "Gotta hurry, only X amount of hours left" or when I get back I'm wondering if I could have done something different such as set the sluice up a different way. Basically what I'm getting at is, we prospectors have to make sure we make the most of our time out in the field because you never know when you will get another chance to go. Between work, family, and LIFE, I have found it hard to get out and prospect lately, so when I do go I make the most of my time.
So how do you make the most of your time? One useful tip I have that may help speed things up for you is the way you classify. Most prospectors own a detector of some sort. I have a White's Electronics MXT. It is a great detector at picking up really small gold. When I am out in the field classifying material before I run it through my sluice, I do not even glance at the bigger material that would not pass through my 1/4" classifier. I have a homemade classifier I made out of a 5 gallon bucket that I cut a whole in the bottom and fastened 1/4" mesh hardware cloth. I simply place this bucket inside another 5 gallon bucket with a little water. Fill with material, give it a couple of twists side to side and dump. I can literally fill a 5 gallon bucket full of 1/4" classified material in about 2-3 minutes. It takes me longer to shovel than to actually classify the material!
So instead of taking 2-3 minutes to dig through each bucket of larger material before tossing out, I burn right through my bucket. Once finished, I then take my metal detector and wave it a few times over my "reject" pile to see if any gold nuggets got caught up. With most any detector, a gold nugget 1/4" or bigger in size will SCREAM in the detector so I have no worries of missing any chunks of gold. I hope this tip will help you speed up things and make the most of your time in the field.