I actually bought caps and made my own stick-in-the-ground poles... drilled a hole in the cap, put a washer inside, ran a 6" screw through the hole, then threaded on a bolt. You could glue the caps to the poles but mine aren't glued and the caps have never come off the poles, even when yanking them out of really hard ground. Do be careful you don't yank it up into your face though, LOL.
I don't use what CaliTerp linked, but I should! I usually just measure with my feet. But yeah, those things are really neat and make it super easy and I should have one, LOL.
ETA: Come to think of it I could MAKE one of those placers... hmmm. Guess I know what I'm doing this weekend.