Keep a Foraging Journal
The practice is simple: whenever you gather a wild food, make a note of what, where, and when you gather (e.g., first Common Dandelion leaves, backyard, 4/15/14). You might also note other plants who are almost ready to harvest or who've gone by.
I've kept records in a spreadsheet document (sample below), with columns for the plant name (both common and scientific), location (if I primarily gather the plant in a particular place), and edible part (in case the plant has another part with a different season of availability), and one column for every two weeks of the growing season. Once a day, I went through the list and place an X in the appropriate column to denote which wild edibles were available.
Another option is to dedicate a calendar -- a wall, desk or pocket-sized daily or weekly planner will work -- and once a day jot down your foraging activities and/or observations.
Maintaining a foraging log, however simple or complex, can pay dividends in future years by reminding you of when various foods are available in your area. Over time, you'll likely internalize many of the natural cycles and come to anticipate the availability of your favorite edibles without consulting your written record.
Do you keep a record of your foraging activities? If so, what method do you use?