If you are at all creative, a freshly poured concrete slab is too good to waste -- it's a tabula rasa that can retain any record you emboss into it for the benefit of future earthlings. When pouring a new driveway or garage floor, more than one family has taken the opportunity to leave handprints and names for posterity. The secrets to leaving accurate records involve timing, and the proper pressure and implements.
Not Yet ... Not Yet ... Now!
The window of opportunity for well formed handprints in a fresh concrete pour isn't very large; the concrete remains soft enough to stamp for only a few hours. Don't jump the gun; immediately after the pour, the surface is too wet to stamp. Approximately two hours later, though, it should have cured and stiffened enough to hold impressions. The time varies according to the outside temperature -- you'll have to wait longer on colder days. Test for readiness by laying a trowel on the concrete, pressing lightly and lifting it. If no concrete moves under the trowel or sticks to it, your window of opportunity has opened.
Trial and Error
It's best to stamp your handprints as soon as the concrete stiffens and while it's still malleable enough to level with a trowel if you make a mistake. There may still be some water underneath the surface, so lubricate your hands with coconut or olive oil to get a good impression and avoid pulling concrete off the driveway. Position each hand just above the concrete and push down with moderate pressure, and then lift your hands straight off the concrete and wash them immediately. Concrete is caustic, and can cause burns if it stays in contact with your skin. If you're unhappy with the print, erase it with a trowel, re-lubricate your hands and try again.
To get your hand impressions to appear in the concrete, you may have to apply a fair amount of pressure, and if a little one in your family is also leaving prints, he may not have enough strength for the task. Maximize the pressure he can exert by having him stand straight, and then bend all the way over to push his hands into the concrete while you support him. You may have to lift him off the ground to allow him to focus all his weight on his hands. Be sure he washes his hands thoroughly after leaving the prints.
If you want to include your signature with your handprints to ensure that future generations have no doubts as to whose prints they are, you should wait for 30 minutes to an hour after stamping the prints. This gives the concrete extra time to stiffen, which will make your writing clearer. You can write with almost any stick you can find on the ground, but you'll get the best results with a plastic ball point pen. Retract the tip or leave the plastic cover on if it's pointed, and write just as you would write on any other hard surface. You don't have to push any deeper than 1/4 inch for the writing to be legible.