İpsala-Kipi border crossing
Walking across the border is not permitted. However, there is lot of traffic and finding a ride is not a problem.
While crossing the border from the Greek side, the policeman on checkpoint might give you the stamp in your passport even if you don't have a ride yet which makes finding a ride much more easier. Zenit however encountered a Greek border guard in November 2011 who wasn't willing at all to oblige (and told him he'd get shot should he try to cross the border on foot). If you get your stamp, you can try getting rides after the duty free shop at petrol station where trucks usually refill their fuel.
Other useful info
Drinks from coffee machine at that petrol station are free.