At times I plant a row and need to know exactly where the seeds have been planted. Usually this means that I need to come back later and apply mulch (heavy in the walkways and just a dusting where the seeds are), or plan to apply water to help the seeds germinate. I don't use sprinklers to water the garden, so there's no way I want to waste water and time watering the walkways.
One of the ways I've come to like for marking rows is sprinkling a bit of flour. Yup. Plain old baking flour. It's nicely visible, safe to use, biodegradable, cheap. Plus it's still visible even after 2 waterings or rains. It's faster to use than running string. It's more visible too. Leave it to me to trip over a string, totally obliterating the row marker. And sometimes I've been the recipient of a slightly moldy or caked bag of flour that's being discarded by someone else. Wow....free stuff....right up my alley!
I think that on an earlier post I was advocating using little sticks and twigs for marking rows. But now that I have a hefty supply of cast off flour, that's what I'm using. I definitely prefer it to twigs.