Mary,

Where circumstances permit, I create an informal raised bed by just mounding up the soil and raking the top flat. Then the bed itself serves to mark where I plant my seeds. If I finish a seed packet, I pour some dirt into it for weight and lay the packet down as a marker.

Rain and weather will disintegrate the packet within a few weeks, so you may want to provide a more permanent marker. For "permanent" markers, I use wooden shims from Home Depot and write on them with Sharpie® Permanent Markers or Sanford Ebony pencils.

ZM

I mulch my beds heavily to prevent weeds, so anywhere the mulch has been pulled back is a sign to me that seeds are planted there. It's a very good idea to water these areas with a hose every day or two until you get germination.
For more permanent markers, I use cheap vinyl window blinds. I take them apart and cut the slats into six inch or so lengths, then mark on them with a permanent marker, which is not permanent if it is exposed to sunlight. I bury it at the base of the plant under the mulch.
One window blind will supply markers for two or three years.